With a bit of searching, I found that you can upgrade from Photoshop 7, CS, CS2, and CS3.  From CS3, the upgrade price shows as $349.00 (US).  Seems a little expensive for a sidegrade especially if you already have PS CS3 Standard.  This is the price for PS 7, CS, CS2 as well.

I only have the standard PS CS3 and upgraded from PS7 for $200.  I was going to get the extended, but didn't know how useful it would be since I use Photoshop mainly for video work (titles, stills), and postwork of stills.

Poser 2, 3, 4, 5, 6, 7, 8, 9, 10, Pro 2014, 11, 11 Pro