"It does not hurt to copy the drive and do the upgrade for free and then after updating, swap the drive again for your Windows 7 installation and then swap back sometime in 2020 (you can do a universal restore image not including hardware as well, just to make sure you can restore to where I can not officially recommend... It is a little work, and may not work with OEM installs, but possible)"
Too much trouble, too little gain. By 2020 Windows 13 or 14 will be out and Windows 10 will be relegated to the pages of history (like Win 286, Dos 4.0, Me and Vista). Or due to their sheer stupidity and arrogance, Microsoft will be Class Actioned into oblivion and we'll all be running Linux or Macjunk.