Regardless, of whether you're going up to a newer OS, or down to an older OS, there are certain long established rules that can not be thwarted.
Data files, like docs, pics, movies, etc. can and should be copied off of the hard drive and onto some other media, like CD's, Flash Drives or external HD's.
Most programs cannot just be moved because they are registered into the system registry, and will have to be installed afresh into the new OS.
So once you have everything off of a HD partition, that you need to save, then it's immaterial what you do with that HD.
The simplest thing to do, is just boot up the PC with a Flash Drive containing the new OS that you want to install, and let the Installer Program reformat the HD for you, and even remove any or all unwanted partitions.
Then once the install is complete and you have a new, running, OS, you can copy all your data back to the HD.
After doing this literally hundreds of times, for myself and my customers, I've adopted a new tack, of never putting my data files on my C: drive. Instead I keep them all in a series of five folders on my D: drive. (the #2 partition of my SSD) I access those files by way of shortcuts to those folders that I keep on my Windows Desktop. And, if I have a program that's going to save files, I tell that program where I want it to put its files.
It takes a little bit of work, and customizing, but it's time well spent to keep my precious data files off of the OS partition that could go away at any moment of any day. I've seen so many data losses in my 40+ years as a Computer Tech, that I guess I've become somewhat Gun-shy.
I also back up my data folders to different external media, that are not sitting right next to my PC.
(I came home one day to find my entire computer system GONE, including the disk bank that was setting next to it. My backup files were safely hidden in another room.)
I do wish you good luck in your endeavor.
Cheers Mate,
TM
