Contract amendment when bill plan complete

Due to Award modification Funds need to move from project 1 to project 2

Project 1 is a fixed price contract has several bill plans and all of them completed.

To move funds we added a new contract line for project 2 and tried to amend contract to move an amount of $77000 to project 2 from one of the bill plan of project 1 which has bill plan in complete status for the amount of $700000.

The contract does not amend with a message that the " contract cannot be amended below the already billed. Either reverse the billing plan before amending the contract line amount or change the amended amount to be equal to or greater than already billed amount"