I cheat a bit and asked this question in the Core #editor-js Slack Channel and got this response from a core developer:
“that’s an interesting one. If I had to imagine, you would offer a per-block transformation routine (similar to the
migrate APIs) that would match every existing block of the template to a new state/different block. Might happen on loading a post, or could be done in bulk.”
I realize this goes a bit beyond the scope of the content current in the course and is a tiny bit of an edge case, but not really.
Would be happy to chat this out, if you wanted to explore a solution around this.
Hope this helps a bit! Good question 🙂