i took one of my servers, and switched distros. I had all the junk i wanted to keep on a seperate drive, and all i had to do was add the drive to the fstab.
it appeared, but the permissions where all jacked up. I got angry and wrote a shell script to fix the permissions issues, but all and all, it worked.
It is possible to do, but my problem was with the old permissions on the drive... so, if you can work with correcting that, I think you'll be alright, provided you are talking 2 different drives... if it's one drive partitioned up, one false move and you're liable to delete everything...