For what its worth, I had to reinstall windows 7 on a Dell laptop that had been upgraded to windows 10, The lady who owns it didn't like win 10 at all so I downloaded Dell recovery suite to get access to the Harddisk recovery partition (which was still intact) made 4 DVD's from recovery drive and goodbye win 10...... and thats after 3 months of use.
As long as you have the original key, there should be no problem going back. but after 29th July I think
any install, even with a captured hardware profile, will be an paid for upgrade?
I have got Win 7 and Win 10 on dual boot on my machine (had spare win 8 retail key) so I can keep my win 7 retail License.
Also had to ring MS to get win 8 key cleared for reuse!