DH: According to the Rambam (in the Moreh) Ikkarei Emunah are immutable. Holding that they can change would be itself a violation of Ikkarei Emunah. Thus, the two Shittahs do not go hand-in-hand.
And writersoul is right and you are just wrong. If the Rambam reaches a conclusion which he then can conclusively prove wrong, then the original Shittah was a mistaken Shittah. If he came to a Shittah based on inaccurate information, then that Shittah is incorrect. That holds true of the Rambam, any other Rishon, or you or me. When he holds of two mutually exclusive Shittos, one is wrong. If the Rambam thinks over a Gemara for a year and reaches a Maskana, and is then Chozer on that Maskana because he didn’t have access to a P’sikta that shows the context of the Gemara, then his Chazarah is saying his original Shittah was based on false information and wrong. So too here.
Also, why do you think the Poskim always say that the Piskei HaRosh is more important than the T’shuvos HaRosh because they came later (maybe I’m backwards?)? Because we care about the Shittos that the people thought of. You can’t have a Machlokes of someone against himself. If he was Chozer then he was Mevatel his earlier Shittah.