[H] two drives, two OSs, one PC

[Winterlight]

I read on PC World that is still possible to use a Retail Windows 7 
or 8 PRO key to clean install free to Windows 10. I had a mini 
computer running windows 7 pro so I tried it and so far it seems to 
have worked.  Windows 10 Accepted the key and installed. Of course I 
won't be sure for a couple of months ... I might get a message from 
MS at any time.

However, since that worked I am thinking of using another windows 8 
PRO 64 bit key I purchased direct from MS and never used. I have 
another mini computer that is running windows 8 Media Center. I don't 
want to loose it quite yet and I do not want to dual boot so my plan 
is to remove the mSTATA drive from the LIVA mini PC and put in a new 
bigger mSTATA drive and then use my unused Win8 Pro key to install 
Win10 pro. Once I do that I will have win10 pro registered to the 
BIOS and it should be good for life. Then, for the time being I put 
my Win 8 with media center mSATA drive back in.When I am ready to 
make the move to Win10 all I have to do is put the Win10 mSata drive 
back in. Or is it that simple.

My only concern is that MS won't allow me to go back to my old mSATA 
with Win8 media center even though the Win 10 mSata was installed 
with a different Win 8 Pro key. What does the collective think...safe 
to do ... maybe ...won't know till I try it?