How do I migrate all users data into a RAID array?

Rsync will preserve permissions(actually cp -p will do this as well)

As per changing everyones home directory, you don't have to do that. You have two options(note, this assumes all users directories are mounted under /Users/ldapusers, obviously substitute the real location in its place)

1. Just make a soft link, issue this command after moving the data
sudo ln -s /Volumes/mybigraid /Users/ldapusers

This *SHOULD* work, though I recommend against it as linking between volumes, esp. for things accessed over the network can get a bit dicey.

2. The preferred method is to change your mount point

http://hints.macworld.com/article.php?story=20080531040605723

I have had a lot of luck sharing stuff out via AFP and NFS over a mounted drive, and I really recommend method #2

Just replace /my/mnt/point with /Users/ldapusers (or wherever you originally had the user files) and you should be golden.