What will happen if I let my Professional Mailing List expire and go back to the 500 subscriber limit?
From Bravenet Wiki
If you have more than 500 subscribers, you would not be able to send any more newsletters until you either a) upgrade your service again or b) delete some subscribers until you have 500. This includes pending subscribers also.
If anyone tries to join while your list is full, they would be notified the limit has been reached.