Its unclear about the version dependency from istio version and AKS version when we read from istio docs or from microsoft docs.
As per the Istio Docs https://istio.io/latest/docs/releases/supported-releases/, its telling that istio 1.7 supports only kubernetes versions [1.16, 1.17, 1.18], and for istio 1.8 [kubernetes -1.16, 1.17, 1.18, 1.19].
But as per the Microsoft docs https://learn.microsoft.com/en-us/azure/aks/servicemesh-istio-install?pivots=client-operating-system-linux)
The following instructions reference Istio version 1.7.3. The Istio 1.7.x releases have been tested by the Istio team against Kubernetes version 1.16+.
So here its not clear that whether any AKS version higher than 1.16.x is compatible for istio 1.7.3?
Just to clear that, if we Upgrade AKS version 1.19.x, whether it will be compatible for istio 1.7.3?
Well Kubernetes 1.19 was released on August 26, 2020, and Istio 1.7 on August 21, 2020. So therefore it might not have been tested against that version.
Still, I guess it will most likely work, assuming Kubernetes 1.19 did not have any breaking changes. Nevertheless, if you want to be on the safe side, upgrading Istio first might be a good idea.