I doubt that your MyBT will show anything about your Talk21 email addresses other than what is showing also I doubt they will be "downgrade" because the email system is set up for BTMail and not legacy email systems such as Talk21.
It might be the case that any "new" email addresses, if you are able to set one up would be subject to the BTMail system and would only be able to be accessed using webmail but that is just speculation on my part.
As it stands just carry on as you are but don't be surprised that if you tried to set your present account up on a new devices the email app that it might not work and you would need to use webmail.
