You can activate a new device that has been used with BIS without wiping it first. BIS and BES are completely seperate and operate independantly of one another. A wipe wouldn't explain how you fixed the policy problem the first time around because a wipe doesn't remove policy - hence all the questions on the forum about how to get the blasted policy off
If you EA a user's device it will populate all of their corporate mail/calendar/tasks etc onto the BB and any entries henceforth will be synced, but any previously existing data will stay on the BB ONLY. It will not propogate to the corporate mailbox.