I have successfully converted 3 PCs at work from Windows 8.1 Pro to Windows 10 Pro with no problems. I spent a lot of time on the PC I personally use, because I had to do a lot of prep work before I was ready...new versions of many different software applications. And I made the same changes at home, knowing I was going to convert that PC to Windows 10 Pro as well. The 3 at work that upgraded OK: a Dell XPS 8700 64-bit, Dell Optiplex 380 64-bit and a Lenovo Flex laptop. The XPS 8700 required the additional work, including upgrading to Nero 2016 Platinum.
So, I did the upgrade work at home in parallel with the work PC, like I said. The PC at home is a Dell Optiplex 990 64-bit. The PC at home has a bit of a history. It was upgraded to Windows 8 Pro from Windows 7 Pro. I was unable to upgrade to Windows 8.1 using the store upgrade path. It would go most of the way and then fail. And I was having trouble at that point because some of the 8.1 upgrade took and most did not. Using some tips from Windows Secrets and elsewhere, I created a Win 8.1 Setup flash drive and managed to successfully clean install Windows 8.1 Pro and recover my applications and data from an image backup I took.
So now, here I am again. All the software applications are Windows 10 compatible. Newer driver for my printer loaded from Dell that I determined would work for that same printer at work. I started with the upgrade tool, which is what I'd been using at work. It downloads and starts trying to process the upgrade and the little box appears telling me the installation has failed. I tried the GetWindows route via Windows Update and that failed as well. I don't remember the error number, but researching it didn't give me any information on what the problem really was. It did roll back successfully to Windows 8.1 each time I tried. I tried uninstalling some apps and disabling ESET Smart Security. I didn't uninstall it, but then, I didn't have to uninstall ESET at all at work and the upgrades went well. So I don't think ESET is the problem. There is something going on with that PC. In the past, I had done "BCDEDIT /set advancedoptions yes" to always have that menu come up that allows easy use of safe mode. I tried to undo that now and could not. Doing "BCDEDIT /deletevalue advancedoptions" errored out because the option wasn't set. If I set the option, rebooted, took off the option, rebooted, that had no effect on how the PC starts up. So maybe there is some corruption in the OS somewhere.
If I can't make this work, I guess I am looking at $199 to buy a Windows 10 Pro key and loading a Windows 10 setup from their ISO file from scratch. I wish I could make the upgrade work at least once. Then I could reload my PC from scratch whenever and be on Windows 10. I'm not interested in reloading Windows 8.1 from scratch just in order to get to Windows 10. And that could turn out to be a lot of work with the same results. At least if I load Windows 10 from scratch, I would have Windows 10...unless there is some deep-seated incompatibility inside of this PC somewhere that just doesn't like Windows 10. If that is the case, it will be staying on Windows 8.1, I guess.