The GS3 license makes no specific mention of "upgrade versions". The applicable text states:
Licensee may use the Licensed Software on only one computer at any one time, however, you may transfer the Licensed Software from one computer to another.
GS3 does come with a different product key, which would generate a different authorization code. Also, GS3 and GS2 can't be installed on the same computer. The instructions say to uninstall GS2, and the GS3 install routine makes sure you DON'T have GS2 installed. Also, GS3 comes with a "clean GS registry" program, so you can be confident that every trace of GS2 is removed from your computer before you install GS3. The install program doesn't do any "must see you have GS2 installed to proceed" checking (the GS2 ownership validation is done at the time of placing your upgrade order).
I would assume the INTENT is for one to pay for a separate license for each computer. Obviously, if you have GS2 already installed and registered on one computer, there is nothing physically stopping you from installing GS3 on a different computer. However, I would assume that if you ever tried to "re-authorize" GS2 it would not be allowed.
I think the intent of GS2 and GS3 being used on the same computer is clear -- otherwise I wouldn't see why one would qualify for "upgrade" pricing.
Well then if a person wanted to install GS2.5 on an older machine, it would be best to do it now, before ordering GS3.0. Keeping in mind though that once a person registers 3.0 the GS2.5 version can probably never be installed and registered again.