Hi,
If wish to migrate with minimum downtime then Replication could be a better option.
In case of disaster recovery clients will be offline unless process completes.
Q. Is it possible to only import policies and let all the clients check back in? That way it's fresher?
--> Yes, it's possible but will have to create group structure manually if not planning to import database manually. This way it's fresher :)
Did you check this article: https://www-secure.symantec.com/connect/articles/hot-move-sepm-one-server-another-server