If you're running self hosted app search, like I am, with a docker compose, I would add Kibana to the docker compose and expose the ports for ES. Than look at how the indexes are organised from App search. Than reindex from the original es cluster to the app search es cluster. Test and push to production.
Haven't had a chance to try it yet, but should work in theory, I'm assuming an 'Engine' in app search maps 1:1 with an elastic search index.
Note: I haven't attempted this, I've added evaluate "app search" to our backlog, but it could be weeks or more before we get there. I'll try to remember to update this with a better solution if no one offers one first.