True "Undo" is, IMO, a pipe dream for all but brand new projects. That will pretty much require you to rearchitect a decent chunk of anything that wasn't built with it explicitly in mind.
In this case, I don't quite understand why stars and forks need to be deleted when switching visibility, since it seems like the two shouldn't be linked. I can star and fork my private repos just fine, so it seems like it was just easier to clear it out than to deal with RBAC at that level.
Many people won't notice some of their stars having disappeared, but many would be surprised to see their forks deleted without their consent, especially those that are not merely a mirror. People can have diverged forks after adding substantial changes without feeding them back to the original for a variety of reasons.
In this case, I don't quite understand why stars and forks need to be deleted when switching visibility, since it seems like the two shouldn't be linked. I can star and fork my private repos just fine, so it seems like it was just easier to clear it out than to deal with RBAC at that level.