Distinguish formal preference and likes. Formal preference is like prior: both current beliefs and procedure for updating the beliefs; beliefs change, but not the procedure. Likes are like beliefs: they change all the time, according to formal preference, in response to observations and reflection. Of course, we might consider jumping to a meta level, where the procedure for updating beliefs is itself subject to revision; this doesn't really change the game, you've just named some of the beliefs changing according to fixed prior "object-level priors", and named the process of revising those beliefs according to the fixed prior "process of changing object-level prior".
When formal preference changes, it by definition means that it changed not according to (former) formal preference, that is something undesirable happened. Humans are not able to hold their preference fixed, which means that their preferences do change, what I call "value drift".
You are locked in in some preference in normative sense, not factual. This means that value drift does change your preference, but it is actually desirable (for you) for your formal preference to never change.
Formal preference is like prior: both current beliefs and procedure for updating the beliefs; beliefs change, but not the procedure.
I object to your talking about "formal preference" without having a formal definition. Until you invent one, please let's talk about what normal humans mean by "preference" instead.
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