The lens solution is clever and, I’m glad to see lenses catching on outside the haskell ecosystem

I think the primary issue you raise comes down to whether or not the peers can be verified to be updated before applying the migration op. If this is the case, you can go the standard sql route of deploying a diff adding forward compatibility, applying migration op, deploying diff removing backward compatibility. This is the route I’ve gone since in my case the peers are web browsers so I’ve got a pretty reliable deployment system; though the lens solution is much better since it lifts this requirement