^ That's a never ending discussion. Does the EULA demand you have an earlier version of OS X? Nope. Does it check for a previous install? No. The retail version doesn't.
I've once had an upgrade version of OS X, I believe 10.3. The G5 I had ordered at the time came with 10.2 but I recieved a free upgrade. The install disk checked if 10.2 was installed, only then would it continue with installing 10.3. I know this, because right before installing 10.3 I decided to wipe my harddrive. 10.3 wouldn't install so I had to install 10.2 again...
That's what I call an upgrade version. The retail one is different, so I can't call that an upgrade version. The name is taken. IMHO that is.