Great what nip number should I use and what kind ?

No need for a replacement event. Just a regular one, signed by all parties. We can even have a contract termination event the parties can sign to indicate they are ending it.

OMG this is great

nostr:nevent1qqstzn22kdn4vkr30cvmgz85q7f9y9wjvuxjgyga0lt3gpv3psn38lgppemhxue69uhkummn9ekx7mp0qgsyvrp9u6p0mfur9dfdru3d853tx9mdjuhkphxuxgfwmryja7zsvhqrqsqqqqqpm204tt