Best way to update host pool VMs with new image

[TomKing Chen]

I have seen a lot documents about creating a new Host Pool using updated image. But not many explain how we can update VMs with the existing Host Pool. This blog indicates it can be done by redeploying the Host Pool with ARM template. I did give it a try, got some syntax error with All Application Group References parameter. After skip the parameter, I was able to susccessfully redeploy the Host Pool. Though users lost access to previously configured Application Groups. In Azure Portal, I can still see these Application Groups associated with the Host Pool, but users can no longer access those Apps. 

Anyone know a better way to update Host Pool VMs with a new image? Or can provide some guidance on how I should setup the ARM template? Especially with All Application Group References field. I tried copy from the Parameter file, which is in the format of {"value":"/subscriptions/.../WVDHostPool-DAG,/Subscriptions/.../AnotherAppGroupName"} and "/subscriptions/.../WVDHostPool-DAG,/Subscriptions/.../AnotherAppgroupName". None of these works. 

[#2]

Have you seen Robin Hobo's more recent blog on this? (https://www.robinhobo.com/windows-virtual-desktop-wvd-image-management-how-to-manage-and-deploy-cust...) That one from June 2020 is also outdated in some regards since the UI has changed, but I think the general approach is still valid -- update the image, put the existing VMs in drain mode, add the new VM(s) to the host pool, and then remove the old VMs. (I'm still learning here, so don't take my advice as gospel!)