I too have this problem with one of my email addresses. This address is one of my 'managed email addresses' so I do not understand why its marked as 'to be downgraded'.
Are you able to assist please ? FYI I do not know the email password for this particular email address.
If you have not been using the email account or if you are no longer a BT Broadband customer it will be downgraded.
To stop it being downgraded you just need to log onto it via a web browser and that will re activate it.
If you can not remember the password you need to follow the forgotten password link found below the log on boxes on the BTMail log on page.