To the best of my knowledge:
What that quote is saying is that permanent damage inflicted to a virtual machine (worst case scenario) caused by any sort of incompatibility between PDM 8 and Yosemite might not be recoverable. This is just a possibility, not something that I have seen reports of.
Each release of Parallels has a 2 year (2 releases of PDM) of updates support, Yosemite was released at the end of this cycle for PDM8, this means Parallels hasn't tested PDM8 with Yosemite, and, as far as I know, PDM8 did not work with the Preview releases of Yosemite, personally, I was surprised that it worked (boots VMs) with the final release (judging by the reports on this thread), however, if Yosemite releases after that break compatibility, users should be aware that Parallels will not update PDM8 in order to be compatible, because its support cycle as ended. Also, because PDM8 wasn't tested with Yosemite, Parallels cannot guarantee that there aren't any short or long term issues with this combination (if it's safe or not), you'll have to rely on reports from other users such as the ones on this thread.
My recommendation is to make/have backups of your virtual machines (or Windows installation if they are Bootcamp based), specially before each subsequent OS X upgrade after 10.10.
Anyway, I'm going to try to answer your questions, but I think they are mostly based on fear and doubt rather than actual knowledge of the situation (sorry if this might offend you in anyway, but I'm a straight talker and very blunt).
"Is this an issue or not?"/"Should I expect this to happen to my users?" - It's a possibility, I haven't seen any report of this actually happening.
"If it does happen what would be the logical course, and if fresh install, then would the upgrade license even work?" - If you have backups (of the Virtual Machines/Windows Installation) then you can roll back. Why wouldn't the upgrade license work? As long as you still have the PDM 8 full license when it asks for it, there won't be any problem if you do a fresh install of Parallels. Or did you mean Windows fresh install? Again if you have backups (VMs/Windows) you should be ok, by using these backups with PDM10.
Last edited: Nov 24, 2014