You’re welcome.

Presumably your logon to my.avast.com is using your previous email and password ?
If so you should still be able to logon using your old email address, it doesn’t actually have to be active to logon. If you can logon, great, then create/add a new email address and authorise it. Now you should be able set that email as the default email and eventually delete the old one.

How did you logon to the forums ?
Presumably not using your old email address as logging on to the forums should use those my.avast.com credentials.