Reversing bill plan

We are on people soft version 9.1, tools 8.55.

There is a contract which is Cost reimbursable but need to be converted to fixed price. It has already been billed and also has some BIL rows. We billed all the BIL rows so now they are BLD then tried to reverse the bill plan and marked it as complete ( bill plan)

but we keep getting the message below.

So can As incurred billing plan not be reversed ? If yes what is the other option to net out the CRB invoice which have been already billed. So that contract can be amended to fixed price? Thank you