I have Taken my Time to review the forum set up and Discovered the only change that can be made to already created account is password reset.
I am suggesting if we can update the forum set up in a way that users can change the email link to their account.
Unless am reading this wrong otherwise the whole topic seems to be based on a wrong assumption! Because it's possible to change the email address from your account settings. So am not sure what the problem is exactly?
If you sell your account and the new owner changes its password, you can't recover it without knowing the new password even if you have access to the email linked to it.
The new owner can change the email too and all you can do is to lock the account from the link on the notification email you will receive.
It is forbidden to sell or buy an already build account in this forum.
No. Buying and selling accounts is not forbidden according to the forum rules. However, it's frowned upon by the community.