Well, if that is the plan, one of two things will happen.
A. Players will abandon the pre 1.8 map for the new features and fun of the new update. The old map being inhabited by a few people, but mostly being a drain of resources until it's removed for lack of use.
B. People will not want to leave their old buildings, making the new map filled with only a sparse number of new players who criticize the old map players for not wanting to get with the new program. Eventually, features will be added post-1.8 that will draw people in from the pre-1.8 until option (b) becomes option (a).
No idea what others would do (I'd move over) but it'd be sad to have to leave my old things, all my old work.
It'd be nice if maybe, we were told to start breaking down everything, make a "vault" in an assigned chunk, and anything in that vault can be moved over to the new map.
How I'd do it, step by step...
1: Go to middle of nowhere and flatten a large area
2: Use world edit to /replace single chunks to alternating dirt/cobble in a checkerboard pattern
-This make a clearly visible way of identifying chunk areas.
3:Allow players to claim a chunk, give it protection, and label vaults with sign.
4:Players build vaults UNDER ground level
-This makes the area still be walkable
5: Once protection/lwc is updated for 1.8, copy/paste the area into the new map
-or whatever process would be needed to make this possible. Not 100% sure how copy/pasting chunks works
6: Re assign protections in the 1.8 map vault chunks
7: Party, because now everyone has their stuff back.
I feel this should appease most players, while no, their buildings cant be saved, their hard mining for rare resources is saved. While still being able to get the new features, clear junk clutter/abandoned buildings, and get a clean world to build our new creations in.
If enough people want to stay on the old map and you want to leave it up, make this option voluntary rather than dismissing the option entirely.