Back up EVERYTHING you have and want to keep to another drive. It might save most of the stuff but you will loose 99% of the installation files and all of your passwords,logins and Browser bookmarks. Win10 uses a combination of Profile based installation locations along with the regular old installation directory.
Example of where it stores the Profile and Mods and Maps for COD4 in win10. C:\Users\"User Name"\AppData\Local\VirtualStore\Program Files (x86)\Activision\Call of Duty 4 - Modern Warfare\players\profiles and it installs the actual game files in Program Files (x86). C:\Program Files (x86)\Activision\Call of Duty 4 - Modern Warfare
Create a Microsoft account if you do not have 1. This will make it much much easier to activate windows. Once you install Win 10 you can simply use your Microsoft account to log into windows and it will activate. After you log in to windows and you verify that it has successfully activated you can then switch to a Local Account so you don't have to put in an annoying user name and password every time you turn your PC on.
Once you are upgraded and it Activates (Make 100% sure that it activates after upgrading) it would be a very very good idea to format the HDD and do a Fresh install of win 10 using the Latest Pre Patched installation media which can be had by using this link.
This is the latest Installation media tool from Microsofts site. https://go.microsoft.com/fwlink/?LinkId=691209
Best thing about the Pre Patched Installation media is that you don't have to sit and wait for hours apon hours of windows updates and restarts. It might have a few small updates but it's much much much muuuuuuuuch faster using the Pre Patched Installation media.
Make sure that you have an empty 4-8gb+ USB Thumb drive handy. Select "Create Installation Media", Next tab,Select Language/Win 10 and 64bit version. Next tab select USB Flash Drive. Select your USB drive and hit next and it will start downloading. I just made a fresh Install drive while I typed this out and it is 3.15gb for Win10 x64.
To do a fresh install you will need to go into Bios and make sure that USB booting is enabled and set boot priority to USB 1st. Or you can press F10* when you start your PC and tell it to boot from the USB drive without going into your bios.
* (It can be DEL, F2, F8, F10 or ESC depending on the motherboard manufacturer and brand of your PC if it's a Pre-built)
You probibly know how to install windows from a USB drive if you did it before but, there ya go.