Hi,
Well, that's an interesting one. I'll try and setup a unit test for this to answer your question in a better way.
Right now I would think that updating the processDefinitionId on the running instances is enough. In addition, the history of the process instances should probably be update too if you want to keep that.
But that could be a simple update statement on the database. I'll come back to this after implementing the unit test.
Best regards,