I don't know who needs to hear this, but I'm getting tired just thinking about the testing path you would need to follow in order to fork mastodon and keep it as an in-place upgrade from baseline.
1. stand up a baseline mastodon instance
2. federate with some peers
3. fill it with realistic data
4. migrate to the fork
5. test the fork
6. test federation
7. repeat across several combinations of mastodon base versions and other fedi servers
8. rapidly, during development