What would the Brisker Rov do with the shito of “Mitzvas Betales L’Osid Lovo” Is that also heresy?
To think that the Torah will change is heresy. To find a reason why in accordance to the Torah we have now things will manifest differently is okay. As long as you can source it in the Torah which was transmitted at Sinai, it can be true.
In your above example, מצוות בטילות לעתיד לבא, I think the Ritv”a explains that it is not referring a specific time when Mitzvos will no longer be applicable, but is referring to a situation. The Gemara darshens כיון שמת נעשה חפשי מן המצוות – Dead people are not obligated in commandments. The Ritva understands this to mean that even after תחיית המתים – the Resurrection of the Dead – those who have been through death will not be obligated in the Mitzvos.
This is not because the Torah will change. If today somebody would be resurrected (as there have been stories recorded in Tanach, Shas), he would equally be exempt from Mitzvos.
EDIT: See R’ Elchonon Wasserman in Kovetz Shiurim Ch. 2, Siman 29