I know that my question is a bit broad and maybe abstract but I'm not sure how to phrase it better. I'm looking for any change in avodah. It can be a change in which group of Jews he associated with (as a modern example Charedi/National Religious/ etc.) or Chassidish/Litvish/etc. I'm just looking for a major shift by a "giant" talmid chacham.
It's said that Rabbi Menachem Mendel of Kotzk, if I'm not mistaken, would study Gemara out-loud before he discovered Hassidism (presumably as you remember it better that way); after his transformation, he studied it quietly (I suspect as you could be learning a page all about your cow eating someone's toxic plants, and using that as a way of personal introspection and meditation on G-d's presence.)
Legend also has it that Rabbi Leibel Eiger became Hassidic, which prompted his father (Rabbi Shlomo Eiger) to sit shiva, but not his grandfather (Rabbi Akiva Eiger).
The Yismach Moshe was one of the misnagdim that switched to Chassidus https://en.m.wikipedia.org/wiki/Moshe_Teitelbaum_(Ujhel)
And for the heretic list, although I'm not sure if it's really what the OP is looking for, Tzadok and Beitus went off the path of truth when they misunderstood Antigonus Ish Socho as brought in pirush hamishnayos from the Rambam in chapter one of maseches Avos, Mishna three. And although they personally believed in neither written law nor the oral law, the schools they formed, the Tzedukim and Baitusim, did believe in the written law and used that path as their vehicle for avodas Hashem.
I think another example might be Rabbi Tzodok haCohen of Lublin who was raised in a Lithuanian rabbinic family and switched to Chassidut. Though one might might argue that he wasn't yet a 'gadol be'Torah' when he made the change. https://en.wikipedia.org/wiki/Zadok_HaKohen