This was on 3.5.0 so I am not sure if this is still an issue (haven't had the desire to test it out again on my live install....) A customer that pays on the 1st of every month upgraded their account during the first month they had signed up (prorate billing), when they ordered the upgrade through the client center it charged them $15 (even though the monthly price difference was $10), and reset their monthly payment date to the 9th. I had to manually fix this in the billing system.