Thanks again everybody for the great answers. Here's a related question that I can't seem to find a current, definitive answer to easily available anywhere else: can I dual boot 7 and 10 in the same...

Thanks. Out of curiosity what will happen if I tried to go straight to 10 with an OEM 7 key? I think 3 out of 4 are OEM, but Win7 installs are quick and easy so no worries there. :-)

Thanks for the response. So it sounds like my plan is mostly good, except I won't be able to harvest the key (nor will I need to). I think I will proceed with the idea of using a spare drive for all...

I have 4 retail versions of Win7 running on various machines in my house. I do not really want to upgrade to Win10 yet, but I also don't want to miss out on 4 free upgrades, and the time counting...