Changing my Apple ID email back to @icloud.com

Hi.

I changed my Apple ID from my @icloud.com to a different email address (@protonmail.com). I changed successfully but after using it for a little (couple hours) I decided to change it back, unfortunately I get this error message. "

I also tried to change it on my iPhone but it doesn't change it either.

I do still have access to the @icloud.com account as it is still listed under the "contactable at" section on my iPhone under my Apple ID (my name) and under my Apple ID online under "reachable at". I am also still able to send and receive emails via my @icloud.com email address.

Is there a cool down period I'm not aware of or is it something else?

Any help, ideas and insight will be greatly appreciated.

Thanks :)