When a contract quantity is changed in mid period, the contract should be billed on next billing batch. The choices could be :
"Next billing period" - preventing automatic mid period billing, as of today
"Daily" - Billing from today til next Contract duration date
"Current month" - Full month invoiced from current month till next Contract duration date.
"Next month" - Full month invoiced from next month till next Contract duration date.
It is necessary to keep track of the quantity invoiced on all contracts. Thereby preventing to invoice the same quantity twice.
The billed quantity should be visible on the contract. Thereby give the ability to see if prepaied "contract licenses" are available.