Crisis Core really doesn't factor in here, since FF characters as they are in KH are more or less completely separate from their original games. Still, I don't think there's much basis here to believe Zack becomes Sephiroth, either way. Notice that Zack first acknowledges the feather before disappearing, as if it were coming down from the sky (which it is) and not from himself (as if he were transforming or some other nonsense). I'm more inclined to believe he was simply taken away to another place.
Furthermore, these little epilogues aren't usually so symbolic, so I'm hesitant to say it alludes to his death (as I've seen a few others assume), either.