Changing Halacha

I will start by saying that this is the method already used by the Sages, who several times recited passages in the Bible in an anachronistic manner. I have also long wondered whether they were doing this as a midrash or whether they really assumed that things were indeed conducted according to the law known to them.
 
Regarding the Rambam, the situation is more complex. In the Rabba of the Rabbis, he speaks of the possibility that the Rabbis have laws from the Torah that were established by the Rabbis earlier, and will certainly reinterpret the Torah, in laws from the Torah and also in the rabbis (except that then one would have to be great in wisdom and minyan). Also regarding laws that are taught in lectures, the Rambam's method in several places is that the absolute majority of lectures (except for three or four, as he says in the well-known reply to R. Pinchas the Dayan on the beginning of the Rabbis' Ishut) are renewed throughout the generations and of course were not given from Sinai. Therefore, he is well aware of the changing nature of the halakhah.
On the other hand, my feeling is that Maimonides' picture is also anachronistic like Chazal's, meaning that he sees the Toshbe's as a tradition from Sinai, but he is willing to accept in principle the possibility of legitimate change within it. He does not claim that this was indeed the case with most of all the laws (at least with regard to laws that are not taught in midrashim, meaning that he assumes that most of them are ancient).
 
Regarding the rule that a person does not consider himself wicked (= a'ama'r) that we are talking about, it is not entirely clear whether it is actually a sermon. In Sanhedrin 9:2 and in parallel, it is presented with the reasoning: A person is a relative in his own right and a person does not consider himself wicked (the disagreement there is only if they make a division about the other part of the testimony). Ostensibly, there is a disqualification of a relative here, and disqualification of a relative is a type of sermon: Fathers shall not be put to death for their sons - in the testimony of sons. But in the first verses there it is not clear whether this is really a disqualification of a relative as such. Beyond the language that suggests that there is an additional unique reasoning here, a relative is disqualification to testify both for a right and not only for a duty. And furthermore, in the case of the testimony of a Ba'd, about a hundred witnesses – and the Katzvah and the Rashba were divided on whether it was real evidence or testimony. Indeed, at least the Ra'vad, who is cited in several Rishonim (see, for example, the Rosh Makot 7:1 and the Ran in Sanhedrin there and others), wrote that a person who testifies about himself is a Ba'd and not a relative, and he divided that the testimony of a Ba'd is not testimony at all, while the testimony of a relative is invalid testimony (the basis of the disagreement is in understanding the conclusion of the Gemara in Makot 6:1, "He who kills will save"). Ostensibly, according to his view, this is a rule that is based on the idea that a Shebad cannot testify and is not a witness at all, and we do not learn this from the sermon that invalidates relatives (otherwise there would be no room to divide between relatives and Ba'd). Perhaps this is the LBM, and then it is certainly prior to his view (he believes that all of the LBM were transmitted from Sinai and there was no dispute about them. See a detailed discussion of this in the Responsa of Chavvi Si' Katsav).
I will conclude by saying that the Rambam has two references to this rule. In one place he speaks of the fear that he is one of those who are going crazy and therefore not trustworthy to incriminate himself (this is what jurists cite in the name of the Rambam, when they use him to invalidate confessions received by the police. Starting with the Miranda ruling, which is well-known in the US), and this is ostensibly against the interpretation of the Gemara, which sees it as close to invalid as invalidating the body (and as the Rambam and the Shula itself write, this is not because of concerns that he did not speak the truth). According to this, we are talking here about a fear of lying and not about invalidating the body as close (invalid even for duty). But in another place the Rambam also presents this halakha as simple.
 
In conclusion, it is not clear to me whether Maimonides requires the readings here or whether he really thinks for some reason that this is ancient halacha. If this is a sermon, then in his view there are very few of them and perhaps this is one of them. But if this is a unique halacha from a commentary or from another source, then in Maimonides' view most of them are ancient (although the Jewish Council in each generation has permission to change and add). And certainly if this is the Central Library.
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Asks:
Following this question, I found an example of a change in halakha in this week's parashat (Amor) and in the haftara in Ezekiel:
 
Leviticus Chapter 21 Verse 2 
For if he remains near to him, to his mother, and to his father, and to his sons, and to his daughters, and to his brothers: 
Rashi: But as for his wife, there is no other wife but his wife.
 
Ezekiel Chapter 44 Verse 25 
And the death of a man shall not make him unclean: for father, and mother, and son, and daughter, and brother, and sister, who have not been with man, shall make themselves unclean: 
 
It seems that Ezekiel understood the word "sharu" as a generalization to the six relatives (not including a spouse), that is, differently from the Sages. It is strange to me that none of the commentators on the Nakh emphasized this point, and neither did the Sages address it.
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Rabbi:
This is not a change in the law. Sharu is his wife, this is a sermon. The plain meaning is that Sharu is a synonym for his relative, and the list of six is only the detail of the relatives. Perhaps Rashi needed a sermon because of the duplication (why did they write Sharu? Let them list the six relatives directly). Yehezkel is suitable for the plain meaning, but the sermon has an independent status.
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Asks:
For the sake of argument, the Rambam can accept that at the time of Achan, etc., the law was different. But since the story of Achan is written in the Bible and constitutes a precedent for generations, the Rambam must explain why this act, even if it was ruled for generations, does not contradict the law we have. From the act of Achan, we learn (for generations) that it is possible to kill someone based on his confession, but only as a temporary order.
Elijah
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Rabbi:
Possible, although a bit hard to accept. If the halakha was different then why was it really done that way? The fact that the act was written down means that it probably really was (unless you assume that the writing is not faithful to historical reality)
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Asks:
It seems to me that what Eliyahu meant to say here is that the policy of Chazal and the Rambam to make valid (like a temporary order) for various actions in the Bible so that they conform to the halakhah, is intended to prevent the common people from making incorrect halakhic inferences directly from the Bible, contrary to Talmudic halakhah. In other words, the Rambam and Chazal were afraid that people would come to say that it is permissible to execute someone based on his own confession and bring evidence of this from a certain case, even though they heard that it is forbidden. But the Rambam and the Sages did not really think that this was the historical reality, and they understood that the law was once different. It is possible that this fear stemmed from the popular perception of the descent of generations, from which it was deduced that all Talmudic law was known to the people of the Bible because they were holier and wiser than the Tannaim and Amoraim (and if all the law is known to them, what they are doing is a great deed that is certainly permissible). Even with regard to cases such as David's sin, which they defended, it is possible that they were done to preserve the perception of the descent of generations among the people, the purpose of which is to prevent a situation in which people in the future would think that they are wiser than the Sages and perhaps come to change the Torah according to their shortsightedness.
Pine
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Rabbi:
This is clearly his intention, and it is possible. However, it seems a bit unlikely to me that the Rambam agrees that the halakha was once different on this matter. If so, why and when did it change? Even if it is possible to change, we need a reason and basis for it. We have no evidence of such a change.
Your words about the fear that they will come to change are strange, after all you assume that changes are really being made, so what is the fear? The fear that it will be done by ordinary people and not by the authorized halakhic institutions? That seems unlikely to me. Then you will teach them "You shall not turn aside" without folk tales (not really convincing about David not sinning) about the greatness of previous generations.
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Asks:
As for why and when the law changed, I suggest the possibility that during the Tannaim period, many new laws were learned through deduction and explanation that were previously unknown (for example, the innovation that a person does not consider himself evil). Before this innovation, there was no halakhic reference to the question of whether or not a person considers himself evil.
Regarding the changes, changes are indeed made, but they usually show respect for the earlier generations and try as much as possible not to disagree with them. If changes are made, then it is in few places and when necessary, and usually most of the "changes" are actually innovations (hykash and sabra) in places where there was no halakhic reference to the matter before (such as the innovation of "No one considers himself evil").
As for teaching them not to deviate, one can disagree on the very interpretation of "not to deviate" itself, and then the whole edifice falls. For that, perhaps a combined strategy is needed. On the one hand, there is the teaching of "not to deviate" and on the other hand, there is the concept of generational decline. Together, these two things provide a good integrated solution that will prevent irresponsible changes.
All of this is of course speculation, if you have a better explanation I would love to hear it.
Pine
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Rabbi:
Maybe. I don't see how this is better or more likely than the simple interpretation in the Rambam that thought that was indeed the case.
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Asks:
I came across Radak's interpretation of the verse in 2 Kings, chapter 3, verse 19.
And you shall strike every fortified city, and every choice city, and shall fell every good tree, and stop all the springs of water, and mar every good grove with stones: 
Radak: And every good tree you shall cut down – because the scripture did not say, “You shall not destroy the tip of a tree except when it is planted in a city,” as the verse says, the reason being that you shall eat from it. And even though the R”z accepted that it is forbidden to destroy a food tree in any place and at any time, the plain meaning of the scripture is that it did not say this except during a siege, and it is possible that it was a commandment for a time like Elijah on Mount Carmel, who offered an offering outside during the prohibition of altars. And in the sermon, when the prophet said to them, “You shall cut down every good tree,” they said to him, “You shall not destroy the tip of a tree.” And you say, “Yes,” he said to them, “This was said about the rest of the nations. But this nation is despised and despised before the Lord, as it says, “And it is despised in the sight of the Lord,” and it is said, “You shall not seek their peace and their welfare. These are their good trees.” 

It is implied from the first possibility that Radak suggests that the law in the time of Elisha was according to the simple version, and only in the time of Chazal did the law change and expand so that every edible tree was forbidden to be destroyed (and not only during a siege).
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Rabbi:
Indeed. Although the term "Kabbalo" implies that this is an ancient tradition. But perhaps this is a linguistic convention.