Disagreeing with someone does not make you an apikorus, no matter who that person is.
That is in regards to having a difference of opinion. In practice, however, normative Judaism sees halacha has having been established in the Gemara, and therefore even if an amora was “wrong” it doesn’t matter, Torah lo ba’shamayim hi.
Therefore you can think whatever you like, but you generally can’t change the halacha.