As long as everything is on one disk now I would buy two disks for external images like Bizman said. Then I would install Macrium Reflect (it's free) and clone from Windows 7 to one of the drives, and then Windows 10 to the other after you have it installed.
My system has all of the Libraries installed to their own disks. My Documents, Pictures, Videos, and so on, plus a large SSD for Steam, Adobe, Autodesk). That way the boot drive can be Windows 7, or 10 and everything I use regularly is still there. If I want to reset Windows 10, which I do about every third month now, I only have to reinstall the programs and then point to the data files, cache, or whatever. That's actually what I did when I "free-graded" to W10 was get everything installed, upgraded, and then reset the OS to get rid of cobwebs.
I still have W7 on a single system disk and used Never10 to prevent the update.
Windows 10 is like Microsoft have a court order to force feed you through a tube. Eventually you forget about the tube, but at that point they can feed you anything.