Today remote process groups (RPG) once established cannot have the target URL edited. If a user wants to change the URL they have to add a new RPG for the new target and change the relationships/connections to point to it. Then they can remove the old one.
That process ensures that there is explicit (user provided mapping) to the new ports available at the new target. However, this is very limiting in templates today and would be very limiting to the effectiveness of the versioned flow registry and porting between environments.
We could make the URL support expression language statements but at present that would be the first non component property (like processor properties) where EL is allowed and we need to have a more thoughtful and consistent approach for that to include things like number of processor threads/etc.. A cleaner and more consistent option is to do this like sensitive properties are being handled in versioned flows which is they are not part of the versioned flow definition but rather on import or when edited we allow the user to set their environment/flow specific values all without making the local flow version dirty in terms of versioned flow management.
So, we should allow the user via the UI and obviously through REST API calls to change the URL. This would require stopping the RPG, changing the target URL (resycing/establishing remote ports/auth/etc..), then starting it. We need to make sure this change does not necessitate a new version of the flow.