Hi,
This is what my opinion for this, CMIIW.
If you migrate from on Prem to Outsystems cloud, as long as the DNS to access the platform server is the same, the current app should be still working (should be, but I cannot confirm this)
But I think you should rebuild the mobile apps and re-submit it to the store, the reason for this, the mobile apps can have different version, because the platform is migrated (the only exception if you bring the entire database from on Prem to the cloud, which is usually unlikely, only important data, and leaving logs and etc. behind).
So for temporary, you should be able to use the existing mobile app, but better after that, regenerate mobile apps and re-submit to the store.
Thanks