They wouldn’t reverse the judgment that the person is Heaven, since that is all that is guaranteed in the decree of canonization. As the old Catholic Encyclopedia explains on this point:
[quote=Canonization]There is no question of heroic virtue in this formula [the canonization rite]; on the other hand, sanctity does not necessarily imply the exercise of heroic virtue, since one who had not hitherto practised heroic virtue would, by the one transient heroic act in which he yielded up his life for Christ, have justly deserved to be considered a saint.
I could see them removing the person from the universal calendar or saying the person’s sins are not to be imitated, but that instead they are an example that even sinners can be saved by grace and mercy. Of course, all saints committed sins to some extent or another (the Bible says even the just man sins seven times a day), and we shouldn’t imitate their sins, but their virtues.