AKS Kubenet migration with ingress-appgateway-subnet

### Type of issue Missing information ### Feedback When upgrading an existing Kubenet cluster to use Azure CNI Overlay using the az aks update command, I run into an issue with the fact that there is an `ingress-appgateway-subnet` associated with the route table. Now the documentation mentions that it will delete the route table, which is worrisome as the application gateway was enabled as a feature of AKS. My migration failed, with an error about a custom route table as this subnet is associated with the AKS route table. There is no information about how I should upgrade from kubenet to CNI when I have the application gateway feature enabled. Assuming that the application gateway ingress controller still needs a route table, then how should this be handled. Either the CNI migration process should handle this properly or additional information must be provided on how to make sure that the application gateway ingress is still working when migrating to CNI. ### Page URL https://learn.microsoft.com/en-us/azure/aks/upgrade-azure-cni?tabs=kubenet#upgrade-an-existing-cluster-to-azure-cni-overlay ### Content source URL https://github.com/MicrosoftDocs/azure-aks-docs/blob/main/articles/aks/upgrade-azure-cni.md ### Author @msftjonw ### Document Id 42557c24-0cea-bfd0-b647-0938b31c54c7 ### Platform Id eaafcf8f-a76f-d6c8-857a-9b14a5a3debe