Easy case: only the .book and .fm files are moving:
If both locations are accessible to some user, open all files (per book) at the old location, then save to the new.
If FM11, you might be able to script this.
If it's imported objects that are moving, that really needs scripting to change all the references.
There is an aftermarket "archiving" app that can do that, but it may make them all local.
We infrequently have a need to do just that, make all imported object references local to the document dir, in ./imports/
I wrote a csh/sed script to find all the paths, and hack the MIFs. It's a kludge I can't give out, and still requires manual steps, but it can be done. FM file paths are really ugly in MIF.