I'm struggling to find any info about this in the documentation so reaching out to the community :-)
I have a Windows 10 1909 Multi session Farm and have duplicated the published image, provided a new name, made some updates and published it. I edited the Farm to use the new published image and it appears to of deleted all the VMs in the Farm at the same time.
Is this expected behaviour? I had expected that this would of been a rolling process. Instead it feels more like a big hammer!
I had the opportunity to do some more testing with this and am happy to report that logged in users were not impacted by changing the image. If I recall correctly the Windows 10 Multi Session VMs that had active users were automatically placed into drain mode. The Windows 10 Multi Session VMs that had no sessions were immediately updated.