It is always a wise decision to make a backup of your files before upgrading to a newer operating system. There is always room for errors during the upgrade process and, sometimes, things go wrong.
When you start the upgrade, you will be provided with options to either keep your files and programs or not, so, if everything goes smoothly, you're good. However, what if it doesn't? So, yes, make a backup of your files first!
Nowadays hard disks are cheap. I recently bought a 1TB refurb at MicroCenter for $18 and a new Seagate 1TB 7200 rpm for $40.
1) As suggested, do a full backup -- "It can't hurt."
2) Buy a refurb (or new) hard disk.
3) Clone your existing drive.
4) Remove the old drive and substitute the clone. Make sure the clone actually works.
5) Do the Win 10 upgrade on the clone.
6) Make sure everything works right on the upgraded clone.
Do not assume that because you try 3-5 programs and they work, that everything is fine. After a week you may find that some program you need "once in a blue moon" either won't run under Win 10 or it tells you the program has been installed too many times and refuses to run -- and the vendor is out of business.
7) Once you are sure everything is fine with Win 10, you can clone the Win 10 disk back to your original Win 8 disk if you wish (for instance, if your clone was a cheap refurb).
8) If it turns out that for some reason you can't or simply don't want to use Win 10, just put back the Win 8 drive.
(And, yes, you can still run either the Win 8 or the Win 10 install (although technically, once you upgrade MS might consider only the Win 10 to be "properly licensed".) However, both will still show as being activated. You won't run into a situation where suddenly your Win 8 tells you it isn't activated.)
9) Theoretically, MS's "undo the upgrade" works flawlessly. However, some people have reported that when they tried it it caused huge problems and they wound up having to do things like completely reinstall Win 7, then run a Win 8 upgrade, then a Win 8.1 upgrade, then reinstall their programs. Most people who have reverted didn't have such problems -- but it's a heck of a lot faster and easier and less headaches to simply put back the drive you removed.
There's something about a Win 10 free upgrade few people understand -- Microsoft's view is that the upgrade is locked to the specific physical hardware. In other words, you cannot move your installation (the physical hard disks that contain your Win 10 installation) to a different computer. Microsoft considers it a "different computer" if it has a different motherboard. Win 10 will recognize that it's not the exact same motherboard (same serial number, same MAC address, etc.) and will not activate. When you call MS to try to activate by phone, you will be told that they consider it a different computer and you'll need to buy a new copy of Windows. (I haven't had that situation arise, but that is what I'm seeing from the tech bloggers who talk to MS.) (Of course, the vast majority of Windows users don't upgrade motherboards, etc., they buy a new computer with a new copy of Windows, so for most people "replaced the motherboard" would never be a problem.)