How to fix Service Contract billing schedule if it has been billed partly

Hello,

We have a few contract where the billing schedule was incorrect at renewal - instead of billing advance in full term, it got a monthly billing schedule. It was caught after the first two month billings were processed and invoices generated. Is there a way for us to change the billing schedule so that we can bill rest of the 10 months in advance at the next billing cycle? The user tried changing the billing schedule by setting the 1st stream to 2 months instead of 12 and adding another stream for rest of the 10 months