Windwows 10 will have to offer more than a free upgrade to get me off of Windows 7. Was on XP until a few years into Windows 7 before upgrading. There is a history of problems with new operating systems and it is not open to the public thanks to the internet. No more upgrades without some knowledge of the problems thanks goodness!
Windows 7 has matured and is very stable. Why upgrade to another version without a long stable entry along with a cheap reason to upgrade? We shall see, but today I see nothing but another security promise as a reason to upgrade. Sorry, but free is not a good reason...
Personally I love Win 7, I've got five Win 7 systems at home, and have no plans to switch them all to Win 10 within the first year. I'll probably run one copy of Windows 10 because I need to stay current for professional reasons.
One of the advantages of using VMs (virtual machines) is the ability to revert to an older installation essentially instantaneously, because the entire system is saved as a single disk file which can be run at will.
So what I plan to do is make a copy of each Win 7 VM, register each Win 7 VM for Win 10 and let it upgrade, then revert to Win 7 and keep running it until I feel like I really can't continue (for example, in 2020 when security updates stop for Win 7), and then switch to using the Win 10 upgraded VM.
I haven't quite figured how to do the BootCamp Win 7 installs on my two Mac Minis. Not sure how easily I can revert installed OSes on those. But there's gotta be a way, even if it's only "dd".