Branch and merge maps

I've run into a situation when making multi-phase maps where I realise I need to add something to an earlier phase.

If I understand correctly, you already track the changes in a version control-like way. I would like is an extension of this to branch a map.

The version control feature my use case would need is something like git's rebase: take the changes made after the branching, and apply them on top of the changes made in the parent map after the branch, making it as if you originally branched from the current version of the parent.