Reusing OEM key is piracy? Really? Says who? And who's the idiot that will activate through phone when is pirating software?
Let say it simple:
I, personally, already 3 times transfer 1 OEM key and there was NO problem. Just calling MS support, explain them that my Mobo is dead and want to use my key on a new rig- don't know what they do (and don't care), but at the end I get activated Win 7. And same rig was upgraded to Win 10 without a problem. And returned to 7- no problem... Believe me, if there was a conflict with EULA supports answer will be "We can't do anything- go and buy another key"...
I CAN'T guarantee that this will work with all OEM keys, but definitely works in some cases- just call MS support (preferably- before installation) and ask, they won't bite you...