According to FAR 49.404, the normal process for a takeover agreement is that the government, the surety and the existing contractor sign an three part agreement that makes it clear that the contract will be completed by the surety and the new general contractor.  So there is no need to write a new contract. All you are doing is terminating the existing contractor's right to proceed that was given under a Notice to Proceed earlier in the process and transferring that right to the surety. 

The alternative to this three party agreement would be to issue a Suspension of Work under FAR 52.252-14.  If OGC wants the original prime on an open contract, then you could issue the suspension until litigation is concluded, then at that point go forward under the FAR 49.404 process.