That's a good question: the answer is: it depends. (a) If it really is a "full retail version" of bryce 4, then it's not an upgrade. An upgrade requires a previously-registered (i.e. paid for and with a serial number) version of Bryce 3D. If it doesn't say "Requires previously registered version" on the box, then it's a full retail version... Bryce 4 might have come down in price to $89, especially since Bryce 5 is coming out soon. (b) if it is an upgrade ("requires previously registered version"), is it Corel-branded, or Metacreations-branded? Did your free version of 3.1 come with a serial number? If it didn't, then the upgrade won't work, because you need a serial number, and the upgrade doesn't come with one, IIRC. I highly doubt that the upgrade policy from Corel covers the free giveaway. If that were the case, then they'd never sell any of the full versions. My guess is that you must buy a full version of Bryce 4, and that the free version of Bryce 3d is useless for upgrade paths. Remember, Bryce 5 is coming out shortly, so I'd suggest waiting it out.