The reply I had from support was;
So just to clarify a bit - account management on The Things Network goes via https://id.thethingsnetwork.org/ - not via the account page on The Things Stack
The reason for this is that you have a singular account for every place on The Things Network (forum, website).
I see from your earlier message ( that you are trying to do edits directly from The Things Stack, but this is blocked because management (password / email) needs to go via https://id.thethingsnetwork.org/
What I propose that you logout from https://eu1.cloud.thethings.network/ , then you login on xxxxxxxxxxxxxxxx via https://id.thethingsnetwork.org/ , and change your email there.
Afterwards, go to https://eu1.cloud.thethings.network/ again, login.
The login via https://eu1.cloud.thethings.network/ will update the email in The Things Stack and allow you to re-use the email for another account.
In general as long as you do the updates via https://id.thethingsnetwork.org/ , you should have a better time.