I have some Sales Orders where, say 6 months into the order, the client increases their fee to a higher amount.

We're currently using Billing schedules to bill this, however, I can't quite figure out how to apply the newly increased fee within the same Sales Order.

For example, say it's a £1000 fee per month being billed and as of month 7, the fee increases to £1500.

I want to update the SO with an additional line of 6 instances of £500 to be included on the last 6 invoices of the Billing schedule, but I can't quite figure out how set this up without it applying from the very first month of the schedule.