When you upgrade to win 10 it doesn't convert licence to a win 10 licence, it adds the ability to use that licence as win 10. You should be able to use the old key that came with PC to install win 8 again. you just can't run 8 & 10 at same time.
I don't know what it means by reserved for oem, its just a hdd. You should be able to wipe it and reinstall win 8 on it. You just need to know the key.
If you install a new copy of 8.1 onto PC and use a different key then what was there before shouldn't have any effect. You could buy a new hdd and new install and it will install perfectly fine, but I would try to use what you have now.
Win 8 oem are tied to motherboard, like every windows before it. Its only win 10 oem that can be moved, and win 10 upgrades as well. Its not tied to hdd though, you can change them without worrying about licence becoming inactive