The Rebirth games are more new games than remakes, they're not just the old levels made prettier, in fact they probably have 80% new content.
This one one people wont even recognise much of the old game at all, I re-played the GB one only yesterday and so far I've seen very little of the old game. The old levels have been completley redone and the ropes at the end have gone.
The GB one has aged badly, its painfully slow and the levels are bland and generic, this from what I'e played so far fixes a lot of thing wrong with the original and its great playing an old school style Castlevania as opposed to the less linear ones that we've had on GBA/DS. Konami can "Rebirth" the other two GB games as far as I'm concerned as this is a success.