Sometimes you want to change a client's billing plan (if a food truck wants to decrease expenses at the end of the summer, for example) and you certainly can. You can do so by editing the Client's settings.
The time to do this is in the month before you want the plan changes to take effect, before the 1st of the month.
Dan's Food Truck is ending operations for the summer. They want to be on their high-use plan through September, but Oct 1, they want to move to an hourly plan.
Anytime in September you can switch their "Next Month's Billing Plan" over to the hourly plan. On Oct 1 they will now start being charged hourly.