To move an Azure App Service to another existing App Service Plan, follow these steps:

1. Ensure both plans are in the same resource group and region.

2. Disable VNET integration if configured.

3. Navigate to "App Services" in the Azure portal and select your app.

4. Under "App Service Plan," select "Change App Service plan" and choose the target plan.

5. Confirm and move the app.

For more details, refer to the official documentation.