you have to go in support system --> support.origin.ogame.gameforge.com/

this the message on the dutch forum info about Retro:

The specifications of the server:
Language: English
Speed: Fleet and Economy: 1x
NON-AGS
No Interceptors, battleships have the "old" price.
No expeditions.
The email address can not be modified.
The password can not be changed.
The uni is hosted on a separate server.
No DM, not officers or other DM features
No advertising banner

that email and password can't be changed , means that a exchange of accounts isn't possible (unless you and old owner take the risk)
But i hope sincerly support can help you

An in universe Accountrade is possible, there we can switch the accounts with password+email of the old account. All other Accountrades aren't supported.