Hello,
there is lots of horror stories around on Parallels 10 + Win10, this is nice that there is actually reported success. I'd like to add some my success report to this.
I had VM with Win 8.1 VM with practically nothing in it. It was my "test" system which I did NOT rely on for anything important (the only Win system I rely on is my business Win7Pro system). This was full proper Win8 license I bought some time ago as faculty of educational institution through MS edu licensing system.
Anyway, so this is simple, well working Win 8.1 VM system with correct license. No real applications there. 32bit.
1. I shut down the VM and made copy of the VM ".pvm" file on one of my external disks. Just in case. I found out, that these simple copies of the VM file is all needed for reliable backup. I keep always two or three older versions on my various external disks... Easy to do, just drag and drop on external drive, replace once in a while. Saved my b... few times 
2. I have updated the Win 8.1 to what Win update was willing to give me (like: three times I run the Windows update) and then I was able to start the "reservation" for Win10.... It came back and told me, that my hardware is not capable of the upgrade. I poked around this Parallels forum and found instructions. In summary: uninstall Parallels tools, download iso with the distribution files from Microsoft, install from that iso mounted as CD (link for iso is above in the OP).
3. I downloaded ISO - I tried to upgrade from 32bit to 64 bit version, but that did not work, so I had to go with Win10 32 bit version.
4. I upgraded to Windows 10 by mounting ISO file (downloaded using Mac side) as CD drive and running whatever the installation program is there from my Windows 8.1 VM. It was actually quick and uneventful install, one of the easiest I have done (I do date back to Windows 3.11 and Mac OS 7...). One or few reboots later, do not remember, Win10 pops up and all seems to be kind of working.
5. Win 10 started with default drivers but running - I installed Parallels tools and they installed fine, added the drivers without complaints. Reboot and most of the stuff seems to work. Well: Coherence is not working for now for Win10. Keep that in mind.
6. Most else seems to work as far as i can say. And Win10 activated without problems, I was expecting that would be the case as this was eligible license running on single system (in one VM).
Few notes:
I went from ~22Gb Win 8.1 VM to less than 12Gb Win10 VM. By now it is more like 13Gb since I installed some software. Still, smaller than expected.
Fast, it works faster than Win8.1 as far as I can say.
Parallels seem to need surprisingly little cpu with Win 10. Nearly idle VM, with only Skype running and connected, and the WHOLE OSX, including Parallels, uses 2-3% total available cpu (my system has 8 cores). In task manager the Parallels process + the VM itself show as running on equivalent of 5-7% of single core cpu load. That is actually really good since just this "https://forum.parallels.com" in Safari is using 2 - 5% of single core cpu ;-)
So my experience with Windows 10 in Parallels 10 (all latest versions) is that they work (except Coherence). It was not totally trivial to install, but also not difficult as upgrades come. I had worse experience upgrading from OSX 10.9 to 10.10 which went crazy and screwed up my disk file system and I had to reformat the disk. And yes, this is very easy configuration with very little complications and my work load for this VM is tiny (mostly Skype for very crazy reason). This VM is mainly test machine in the end and I may, eventually, upgrade my work VM system in few months or so when I am sure all is actually working.
Hope this helps. In case you want to upgrade... Do yourself a favor and make sure you have backup of the VM .pvm file on external drive somewhere before you try upgrading. If anything goes wrong, you can always just delete the one which failed, copy the backup back and start again.
Last edited: Aug 13, 2015
