Answer

I should begin by noting that this method was already common among the Sages, who more than once interpreted passages in the Hebrew Bible (Tanakh) anachronistically. Even regarding them I have long wondered whether they were doing this as midrash, or whether they really assumed that things had in fact been conducted according to the law familiar to them.
 
As for Maimonides, the situation is more complex. In chapter 2 of the Laws of Rebels, he speaks about the possibility that a religious court may change Torah-level laws established by an earlier religious court, and of course reinterpret the Torah, both in Torah-level law and rabbinic law (except that in such a case it must be greater in wisdom and number). Regarding laws derived through exegesis as well, Maimonides’ view in several places is that the overwhelming majority of derashot—aside from perhaps three or four, as he writes in the well-known responsum to Rabbi Pinchas the judge about the beginning of the Laws of Marriage—were innovations introduced over the generations and certainly were not given at Sinai. If so, he is fully aware of the possibility of change in Jewish law.
On the other hand, my sense is that Maimonides’ picture is also anachronistic, like that of the Sages. That is, he sees the Oral Torah as a tradition from Sinai, but is willing in principle to accept the possibility of legitimate change within it. He does not claim that this is what in fact happened with respect to almost all laws (at least regarding laws not derived through exegesis, it seems that he assumes they are almost all ancient).
 
Regarding the rule that a person cannot render himself wicked, which is what is at issue here, it is not entirely clear whether this is actually an exegetical derivation. In Sanhedrin 9b and parallels it is brought with the reasoning: “A person is related to himself, and a person cannot render himself wicked” (the dispute there is only whether we apply split testimony to the other part of the testimony). Prima facie, there is a disqualification here based on being a relative, and the disqualification of relatives is a kind of derivation: “Fathers shall not be put to death because of sons”—meaning, through the testimony of sons. But in the medieval authorities (Rishonim) there it is not clear whether this really is the ordinary disqualification of a relative. Beyond the wording, which implies that there is some additional unique rationale here, a relative is disqualified from testifying even to acquit, not only to convict. Also, in monetary law, a litigant’s admission is like a hundred witnesses—and the Ketzot HaChoshen and Rashba disagreed whether that is evidence or actual testimony. And indeed, at least the Raavad, as cited by several medieval authorities (see for example Rosh on Makkot 7a and Ran in Sanhedrin there, and others), wrote that a person testifying about himself is a litigant, not a relative, and distinguished between the testimony of a litigant, which is no testimony at all, and testimony about a relative, which is invalid testimony. (The root of the dispute is the understanding of the Talmud’s conclusion in Makkot 6a, “shall the murdered person save him?”) Prima facie, according to his approach this is a rule grounded in logic—that a litigant cannot testify and indeed is not a witness at all—and it is not learned from the derivation that disqualifies relatives (for if it were, there would be no room to distinguish between a relative and a litigant). Perhaps this is a law given to Moses at Sinai, in which case it is certainly ancient according to his approach (he holds that all such laws were transmitted at Sinai and no dispute ever arose concerning them; see a detailed discussion in the responsa Chavot Ya’ir, no. 192).
I will just conclude by noting that in Maimonides there are two treatments of this rule. In one place he speaks of the concern that perhaps the person is insane, and therefore is not believed to incriminate himself (this is what jurists quote in the name of Maimonides when they rely on him to disqualify confessions obtained by the police, starting from the famous Miranda ruling in the United States). That is seemingly against the plain sense of the Talmud, which sees him as a relative disqualified intrinsically (and as Maimonides and the Shulchan Arukh themselves write, this is not because of concerns that he did not tell the truth). According to this, the issue here is concern for falsehood, not an intrinsic disqualification like a relative’s, who is disqualified even for conviction. But elsewhere Maimonides also brings this law in its straightforward sense.
 
In sum, it is not clear to me whether Maimonides is expounding these verses here, or whether for some reason he really thinks this is an ancient law. If this is an exegetical derivation, then according to his approach there are very few such cases, and maybe this is one of them. But if this is a unique law based on logic or on some other source, then even according to Maimonides almost all such laws are ancient (though every generation’s religious court does have authority to alter and add). And certainly so if this is a law given to Moses at Sinai.
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Questioner:
Following up on this question, I found an example of a change in Jewish law in this week’s Torah portion, Emor, and in the haftarah in Ezekiel:
 
Leviticus chapter 21 verse 2
“Except for his close relative who is nearest to him: for his mother, for his father, for his son, for his daughter, and for his brother.”
Rashi: “Except for his close relative”—his “close relative” means only his wife.
 
Ezekiel chapter 44 verse 25
“They shall not come near a dead person to become impure, except for father or mother, son or daughter, brother or sister who has not had a husband; for these they may become impure.”
 
It seems that Ezekiel understood the word “his close relative” as a general term covering the six relatives, not including a spouse—that is, unlike the Sages. It’s strange to me that none of the commentators on the Hebrew Bible pointed out this issue, and the Sages also did not address it.
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Rabbi:
That isn’t a change in Jewish law. “His close relative” meaning his wife is an exegetical derivation. The plain meaning is that “his close relative” is simply a synonym for “his relative,” and the list of the six is just the specification of those relatives. Perhaps Rashi needed the derivation because of the redundancy (why write “his close relative”? They could simply have listed the six relatives). Ezekiel fits the plain meaning, but the derash has an independent status.
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Questioner:
In my humble opinion, Maimonides could accept that in the time of Achan and so on, the Jewish law was different. But since the story of Achan was written into the Hebrew Bible and therefore serves as a precedent for later generations, Maimonides has to explain why this act, even if it was ruled for future generations, does not contradict the law we have. From the case of Achan one learns for future generations that one may execute someone based on his own confession—but only as a temporary ruling.
Eliyahu
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Rabbi:
Possible, though a bit hard to accept. If the law was different, then why was it actually done that way? The fact that the act was written down suggests that this is indeed how it really was—unless you assume the writing is not faithful to historical reality.
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Questioner:
It seems to me that what Eliyahu meant to say here is that the policy of the Sages and Maimonides to give forced interpretations to various acts in the Hebrew Bible so that they fit later Jewish law, has the purpose of preventing ordinary people from making mistaken halakhic inferences directly from the Hebrew Bible, contrary to Talmudic law. In other words, Maimonides and the Sages were concerned that people would come and say that it is permissible to execute someone based on his own confession, and bring proof from the case of Achan, so they taught us that it is forbidden. But Maimonides and the Sages did not really think that this was the historical reality; they understood that Jewish law used to be different. It may be that this concern arose from the popular idea of the decline of the generations, from which it follows that all Talmudic law was already known to the figures of the Hebrew Bible because they were holier and wiser than the Tannaim and Amoraim. And if all the law was known to them, then what they did constitutes an authoritative precedent and is certainly permitted. Similarly, with cases like David’s sin, where they defended him, perhaps that too was done to preserve the public conception of the decline of the generations, whose purpose was to prevent a situation in which people in the future would think they were wiser than the Sages and perhaps come to change the Torah according to their limited understanding.
Oren
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Rabbi:
Obviously that is what he meant, and it is possible. But it still seems somewhat unlikely to me that Maimonides agrees that in this matter the law was once different. If so, why and when did it change? Even if change is possible, there has to be some rationale and basis for it. We have no evidence of such a change.
What you say about the concern lest people come to make changes is strange, because you are assuming that changes really are made, so what exactly is the concern? That ordinary people will do this rather than the authorized halakhic institutions? That does not seem likely to me. So teach them “do not deviate” without folk tales (not especially convincing ones, about David not having sinned) about the greatness of the earlier generations.
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Questioner:
As for why and when the law changed, I would suggest the possibility that in the period of the Tannaim many new laws were learned by way of analogy and reasoning, which had not previously been known—for example, the innovation that a person cannot render himself wicked. Before that innovation, there simply was no halakhic discussion of whether a person can or cannot render himself wicked.
As for changes, true, changes are made, but usually the earlier generations are given respect and people try as much as possible not to disagree with them. If changes are made, it is only in a few places and when there is a need, and usually most “changes” are really innovations—analogy and reasoning—in places where there had previously been no halakhic treatment of the issue (like the innovation that a person cannot render himself wicked).
As for teaching them “do not deviate,” one can always dispute the interpretation of “do not deviate” itself, and then the whole structure collapses. For that reason perhaps a combined strategy is needed: on the one hand teaching “do not deviate,” and on the other hand the concept of the decline of the generations. Together these two things provide a good combined response that will prevent irresponsible changes.
All of this is of course speculative. If you have a better explanation, I’d be happy to hear it.
Oren
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Rabbi:
Maybe. I don’t see how that is preferable to, or more plausible than, the simple straightforward reading of Maimonides—that he thought this is indeed how it was.
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Questioner:
I came across Radak’s interpretation of the verse in II Kings chapter 3 verse 19:
“You shall strike every fortified city and every choice city, every good tree you shall fell, all springs of water you shall stop up, and every good field you shall ruin with stones.”
Radak: “‘Every good tree you shall fell’—for Scripture only said ‘you shall not destroy its trees’ when laying siege to a city, as the reason states, ‘for from it you may eat.’ Even though our Sages received that in every place and at every time it is forbidden to destroy a fruit tree, the plain meaning of the verse is that it says this only in a time of siege. And it is possible that this was a temporary command, like Elijah on Mount Carmel, who offered a burnt offering outside the Temple at a time when private altars were forbidden. And in the derash, when the prophet said to them, ‘every good tree you shall fell,’ they said to him: But does it not say, ‘you shall not destroy its trees,’ and you say this? He said to them: That was said regarding the other nations, but this nation is lowly and despised before God, as it is said, ‘this is but a light thing in the eyes of the Lord,’ and it is said, ‘you shall not seek their peace or their welfare’—this refers to their good trees.”

It seems from the first possibility Radak raises that the Jewish law in Elisha’s time followed the plain meaning of the verse, and only in the time of the Sages did the law change and expand so that all fruit trees became forbidden to destroy, not only during a siege.
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Rabbi:
Indeed. Although the term “received” implies that this is an ancient tradition. But perhaps that is just a conventional expression.