ExpertAmateur :
So if I am not eligible, I will need a serial key and have to install via the USB method right?
that was the rule last year, when I upgraded. now it may be different, maybe not. I think the best option for you is to use the tool that upgrades windows without usb. it's better, if it's not possible to upgrade, it will tell you. using the USB method it won't tell you anything. besides, using the USb method, it will give you 2 options if I am not wrong. one is to upgrade to windows 10, the other is to remove all and install windows 10. if you choose upgrade, if will tell you to restart the computer and use the tool I am refering, the one called "updated now" on MS website... so try it first, you don't need anything, that tool will just copy the files to your harddrive, restart the computer and start upgrading. imagine that tool as a virtual pendrive...
try that tool first, if it fails or something than use the USB method.
and be sure to save that license key from windows 7, if some problem ocurrs, like windows 10 not activating and becoming a trial version, you can downgrade. there's an option on windows 10 to downgrade to previous versions of windows, you may need it if you run into this or another problem.