Hi, I just built a PC for my nephew and installed window 7 ultimate trial for him. I want to buy Window 10 to upgrade, but unsure which version will work with it. I leaning toward home, because it's a lot cheaper. Do I need to reformat the hard drive if I get the home version? Sorry if it's a dumb question, but when I try to google it only show results about the free upgrade which I can't get.