Plan change can incur incorrect charging for data usage

[ckb6]

Changing a plan will reset the customers data cap to the plan data cap. If the customer has already gone over that data cap, at the next accounting update, they will be charged for all the usage above the new data cap to there current usage.

This is bad however I am unsure of what the correct action is, perhaps set the data cap to the max of current usage and new plan's data cap, or force plan changes to only occur during a rollover???