One other important note - for those of you who upgraded Win 7 / 8 / 8.1 to Win 10 and need to do a reinstall, if you have an OEM machine (like a Dell or HP branded box), and you decide to factory reset it because your OS is screwy, it'll roll you back obviously, but since you activated the upgrade prior to the July 29th cutoff, you will be able to download and reinstall Win 10 again with zero problems.
It will activate normally and you'll be good to go. I am not sure if you can go direct to Win 10 using the above method and relying on the on board BIOS OEM key to authenticate, but I'm about 98% sure it'll work (and I'm too lazy to google that atm).