well.. if it's win7 retail, you are supposed to be able to install it into another machine.. (provided you are not using it on more than 1 at the same time)... as opposed to oem, where you can't. that bit (ie what happens after win 10 upgrade) was never really really made clear. could be because it's the same machine that causes it to raise up a fuss. i don't think ms is allowed to get rid of the rights of people to migrate their win 7 retail.
upgrading win7 retail doesn't give people a win10 retail key. it only upgrades it for that machine and as such, there's no key per se (aside from whatever is generated base on your hardware and stored in ms' servers)
clean install of win10 threshold2 (based on win7 key) or win 7->win10 makes no difference license wise. you are merely skipping a step physically if you are doing a clean install that way (there's no clean install using win7 key with the original win10.... and the freebie period was over with the win10 from earlier this summer, so win7 key is not supposed to apply to that... don't even need the win7 key for a win10 reinstall)