So I can download an .iso file and use my Windows 8.1 key --- is that correct?
Yes, exactly.
I hate clean installs too because it takes months to get everything back again. I always keep the old SSD handy but disconnected in my tower so I can go grab anything I missed. I have backups up the wazoo from Acronis True Image. I need to make a rescue disk for the latest edition of Acronis though.
But I usually buy a brand new SSD and put the OS on and then install programs and grab their data from the old SSD and don't mess with restoring the backups. That way I get rid of old accumulated little problems that seem to build up over time and you never notice them until something goes haywire.
That's a good strategy. Starting from zero would iron out all the niggles from the previous installation. Though, as you stated, it is an annoyance to get back everything all over again.
I guess, we would be performing clean installs once every six months with a major upgrade coming up during July and November.
EDIT: Just check those links (thank you very much) and I think I better buy a new SSD first. Means more research to find the best and fastest SSD's --- think I will get PCI-E this time they seem to be much faster.
PCI-E generally tends to faster, since M.2 uses the SATA controller. But, I have seen some OEMs like Samsung who use controller chip in their drives which makes M.2 as fast if not faster than their PCI-E counterparts. Case in point would be the Samsung 950 Pro vs the Intel 750 series.
But, it would again depend on the connections on your motherboard. Whether the M.2 slots are shared or not. Also, if you're getting the PCI-E one, get one with a high bandwidth. Most of them in the market are the low bandwidth ones.