change username between accounts controlled by the same email
Pretty much as described. I had someone enquire about this because they had one account with their current on stream name and while looking to find their rebranded name they registered it on a their stream testing account.
Since both account are registered to the same email address it would be nice to "switch" the usernames when the rebrand goes live. As it stands now they are stuck waiting while the name goes back into the name pool and hoping the name is freed up for the "main" account.
(and no one takes it during this time...)
Rebranding can take time and it usually isnt something a broadcaster wants to do in pieces (known as new name on twitch, old name on social media platform x etc) being able to reserve or switch usernames in this manner will allow broadcasters to properly plan and release a rebranding.