Tons of web services use progressively higher account limits based on a pay scale. Take Basecamp, for instance: http://www.basecamphq.com/signup
My question is: how does your company deal with users downgrading their accounts to a level that they are already over?
For example: someone has a Basecamp Premium account with 60 active users and 5 GB stored on the servers, and then decides to downgrade to a Plus account.