Angels have the Phone Box
You're quite welcome. All qualifying OSs can be upgraded during the first year for free. So, yes, your retail licenses are "up-gradable" for free for the first year. Genuine W7 + W8 OSs qualify for the free upgrade, it matters not OEM or retail.
Your W10 would be a retail upgrade license to a retail qualifying license, and so it would require an additional step of installing the qualifying OS first, then upgrading. Now, one caveat here: After the first year of release passes, I don't believe it would be free any more to try and upgrade your qualifying OS.
Once a computer has been upgraded and activated, you are free to clean install as many times as you need/want. The activation is a hash/code generated from some of your major system components, and stored on MS servers. When you clean install, you just "skip" when it asks for a keycode. Just remember, "upgrading" something like your motherboard, will require a call to MS to get activation sorted again, as it is then considered a new machine. If you are past the 1-year mark, you may be looking at purchasing a W10 license. However, like-for-like replacement due to hardware failure does not fall into the "new machine" category.