Windows 8 install with upgrade disc?

Hey guys, so if you've been following my little blog you'll know that I have actually finished building my first computer! I'm on a tight budget and I'm trying to get Windows 8 for my computer. So my question is this. If I buy and upgrade version, can I still install it on my computer that is running Ubuntu? It seems like a dumb question, but what I can get from this website is that if you do a little trick it should work?
Trick

Has anyone tried this? And if so is this illegal to do? I don't want to break any rules.

MS sold the upgrade to people running XP, V and 7. People running XP couldn't do a "upgrade" install anyways, you would always have to do a fresh/clean install.

So the code system from Microsoft does allow you to activate clean 8 installs with upgrade keys. In odd cases I've had to call the 800 number, answer the stupid questions and they verify it for you.

From a technical perspective, it's against what Microsoft intended.

On the other hand, when they offered the $15 Windows 8 for people who bought PCs before the end of last year, it was specifically stated in the offer that you could use the license on other PCs and you didn't have to use it on the PC you bought.

So it falls under no one will persecute you and technically MS may have allowed it anyways.

Thanks for the reply! I just ended up playing it safe and bought the system builder discs. Got them on sale on Newegg.