Hi,
I made the switchover once 2 years ago but I lost all my stuff.:-( I inserted one of the disk into the 325 but it could mount it. It handled as a new one.
Probably it can be mount it via linux somehow...
But probably work if you insert the blank disk into the 325, then copy everything from the 323 to 325 and finally insert the old disk also to 325 and make the raid.
Of course before everything make a backup!