Is my copy of windows 7 home premium legal?

I use to have windows 7 starter when I saw this video on youtube on how to upgrade to premium for free. All you have to to do is copy and paste the license key into the anytime upgrade box and it should work. I did this and to my surprise it did work. I didn't use any hack tools or anything of that sort. I even used the microsoft genuine checker tool and it says i'm running a genuine copy of windows 7. However my conscious tells me i did something wrong. Was this legal or illegal, if so how can I reverse my copy of windows down to the original.