I believe you need to find the BBID sign up, on BlackBerry.com, look through it and you will be able to reset. I believe it has to send you conformation email and such as proff its you but it can be done.
If you registered your PlayBook before, you only have to change the details from the webpage and the PlayBook will automaticly refresh the ID when acessed from the PlayBook Options, it looks at the PlayBook PIN.
You can see the devices in the account details from the webpage; https://blackberryid.blackberry.com/bbid/login/