Okay, well that went swimmingly.
Installed 6.0beta3 onto fresh hardware (of older vintage than the client's, but still good stuff), added 10 user accounts, added an ibay and modified it. Logged in to webmail as all the users and sent each other heaps of emails. All good.
Performed the upgrade over the top of the beta3 install (not using backup/restore method, just blast and replace). Install routine warned that it was an older version than it could upgrade from, OKed that, and let it go and do its thing. It did its thing... I logged in to the server and... all the stuff was there. User accounts were transported, ibay was transported... happiness was achieved.
I am yet to perform a restore from backup (will have to be a network backup as the client's box has no tape drive). That is probably for tomorrow, though.