I wasn’t aware of this until yesterday when I took a leap of faith and tried it out. I had to update my Machine Catalog with a new image but in the last step of the update my two options are to either reboot all my VMs now and update them, or wait for the next shutdown. The latter is what I want but I don’t have the Citrix Connector for SCCM (nor do my VMs have SCCM agent installed on them).
My delivery groups are set to reboot every weekend and since that was a few hours after I was doing the update above, I chose to go with updating on next shutdown. Then I took a look today and sure enough when the machines rebooted they picked up the new image. Nice!
Anyways, turns out this is well known info. Found this forum post where someone confirms this, as well as a blog post on how someone’s scripted this. Looks like this only happens if you reboot the VMs from Citrix. So a reboot from within Windows (or whatever OS you are running) won’t update. It has to be initiated from Citrix.