Thanks for using our answers forum! I understand that you are interested in changing the Sling Account (or adding another account) that a Slingbox is currently on without having to re-run setup. You can actually add the already configured Slingbox to another account just by adding it in the Slingbox directory (with the Slingbox ID and either the admin or guest password). I saw that you mentioned that guest access is not enough, so instead of using a guest password when adding the Slingbox to another account you can use the admin password and have all of the privileges.
Here is an article which should assist you with this endeavor:
Hope this helps!
The Sling Moderation Team