I believe the question is enough. Also, is conversion between schools considered apostasy? (I suppose conversion to denominations not universally accepted as Islam could be considered apostasy, such as Ahmadiyya.)
I'd qualify this as opinion-based. If you left denomination A to denomination B, while A considers B as non-Mulsims of course your former brothers from denomination A would consider you as an apostate while those of denomination B would consider you a convert/revert. And that depends on denominations and the relation between them...
As for Ahmadiyya there seem to be consensus among both sunni and shi'a scholar that this denomination is out of the folds of Islam.
"In 1974, the League declared that Ahmadis and related groups are out of the fold of Islam, banned from the Holy lands and should be boycotted socially, economically and culturally" (Wikipedia: Muslim World League)
Their main reason for this declaration is that Ahmadis don't seem to agree that Muhammad () was the last messenger and prophet.