Anon, chazal had the ability to make gezeros. Geonim made takanos. No one took away from halacha, only added on chumros or fences around it. Davening 3x a day was a takanah from the anshei knesses hagedolah – that’s not called a change in halacha.
When the halacha is, for instance, to keep a 2 day YT outside ererz yisroel, and conservative say that nowadays you don’t need to, while the gemara was clearly aware of the fact that there’s no sfaikah d’yoma, is changing halacha.
A man not being able to divorce a wife against her will is the halacha. No one changed that din. What the geonim did was make a takanah on top of that din, saying that you’re not allowed to do so – beis din also has that right, because kol mekadesh, adaatah derabanan mekadesh. Same goes with certain forms of kidushin.
A change in halacha would be if someone said that marriage doesnt require 2 kosher witnesses, which is exactly what conservative did, and which is why rav moshe says we’re not even choshesh for their kidushin, because you need witnesses for kiyuma, not just rayah. Conservative saying that women can be witnesses is also changing halacha. An example of a non-change would be to invalidate witnesses for breaching communal norms, where rabbis would have that authotrity.
Re, k’sav ivri and k’sav ashuris – stop reading things online. the seforim do not call it “paleo hebrew” – you got that from some apikores on the internet or wikipedia.
What those heretics omit is that In the gemara there is a machlokes if k’sav ashuris was there from the beginning, and both ivri and it were used, or if ashuris was only for the aseres hadibros, was then forgotten, and brought back during bayis sheni. Either way, everyone agrees that ashuris was not entirely new and unheardof.
and for your claim about there not being “religious rituals” outside yerushalayim….do you think jews didn’t put on tefillin outside yerushalayim? I have no idea what you’re talking about. Karbanos? They were brought on bamos until the beis hamikdash was built; this is based on pesukim and I have no idea how changing circumstances equals a change in halacha.