Backup ps3 either by backup utility or copying stuff individually...
Deactivate machine under all users / psn IDs...
Create new user and delete all other users...
"Restore ps3 system" to format it and return it to factory settings in one go...
Job done!
(did this a few times as I had a ylod machine followed by a faulty slim...

)
If you take a backup and restore it to a different machine you'll lose any protected game saves, and some downloaded psn stuff, but there's not much you can do about the protected saves, and psn stuff is redownloadable from the store. You might have your albums for pictures / videos messed up too so you'd need to go through them again reassigning their album tags.
Obviously the data transfer utility is the best option but if you won't have the new one and the old one in the same room for a day the backup / restore does a pretty good job imho...