WVD Template to Modify a host pool with new setup - "first time use" - can I use only this template?

Johan_Eriksson : To clarify, is the main goal to say that you have a host pool, but now want to push out a new image? If so, then we would actually recommend the "Update existing host pool" template, which gives you extra controls on what to do with the previous virtual machines.

To answer your later questions on "only updates my resources as required" (such as adding 1 or 2 VMs to your existing host pool and referencing the same image), the goal is that it should work. ARM does this pretty well with VM's, such that it won't create "vm-0", "vm-1", etc. if they already exist. However, ARM is a little less friendly in terms of the extensions, which we need to run to get the VMs joined to the domain and registered to the service. We are investigating this.