Move Azure Web Apps into new Azure App Service Plan?

I have two Azure web apps that are in the same resource group but have two different app service plans.

web-app-1, in resource group web-app-group, has app service plan web-app-1-plan

web-app-2, in resource group web-app-group, has app service plan web-app-2-plan

I created a new app service plan called “web-app-3-plan”, in resource group web-app-group, which was created after the two web apps. Now I want to move both web apps into the new app service plan. All three plans are in the same region (West US) but have different pricing tiers (which shouldn’t matter).

When I go to web-app-1 and select “Change App Service plan” I only see the current plan in the dropdown list and an option to create a new plan (that starts with ASP-).

How do I move my web apps into “web-app-3-plan”, my new plan?

I tried to replicate your issue, and I was able to move the Web App to New ASP(App Service Plan) without any issues. Attached the screenshot for the same.

And after investigating your issue further,

• The reason this might be occurring is because the App Service Plan is tied to a different webspace than the original App Service Plan of the web app, even though the newly created App Service Plan was moved to the same Resource Group.
• What you can do to resolve the issue is, delete the newly created App Service Plan, and then create the new App Service Plan in the same Resource Group rather than moving it to the Resource Group( if you have done it) . This will ensure the App Service Plans are tied to the same webspace (on the same Resource Group and the same region).

Please refer to the below doc for more reference : Move an App to another App Service Plan