Changing app service plan should be improved
If you are allowing to assign app service to any service plan within same subscription (regardless of the resource group), you should be able to change it to any service plan as well.
Luke Chung commented
Absolutely! Also need to easily transfer to another subscription when a solution is handed over to a client.
yes, or disable option to move to other resourse group for app service too
Michel Zehnder commented
Please allow the move between resource groups in the portal