Yeah its a tricky one! I have not had to actually had to execute on this but conceptually i have a solution in mind.
You basically need to maintain an 1.2 version of your “app” and a 1.3 version (so different end points) for a little while, upgrade and republish a new major version of your cordova app (on 1.3), so anyone who is proactive about app updates will move to the new platform.
Then for anyone who doesn’t update you can add a nag screen to your 1.2 code base which will be hot deployed to them, the nag screen will prompt them to upgrade (make it annoying so that it forces the to take action)… Once they all have upgraded you can decommission the 1.2 implementation completely.
This (clearly) isn’t the only solution and depending on your app and your customer base it might not be the most appropriate but for me (conceptually at least) it works pretty well.
All the best