I tried moving an App Service to a new app service plan and faced a similar issue.

I've referred this doc to know how to manage the app service plans.

• As you mentioned in your comment, I tried to move my app to a new app service plan in a different resource group, but I couldn't find the service plan in the dropdown.

• Below, you can see that I've created app service plans with a different resource group, a different region, and a different operating system.

• But I can only see the app service plans that are in the same resource group, within the same region, and with the same OS type in the dropdown box.

The app service plan was changed successfully.

• So, we need to ensure that the target app service plan has the same resource group, region, and OS type as the source app service plan