To register a Slingbox to a different Slingbox Account, all you need to do is factory reset the Slingbox, and get it set up while logged in under the new account. You do not need to delete the Slingbox from the account, or delete your account to get this done.
I'll include how to delete a Slingbox from your account, just for reference. Here's the article that covers it:
Just scroll to the "What else does the Directory tell me?" section, and it'll tell you how.
Hope this helps!