Had a message from a friend in a Design house. He also has CS2 and bought the Photoshop upgrade alone. When installing the installer said it couldn't find the application - it was asking for a version of Photoshop, not the entire Suite. When he typed in his Photoshop serial number it was rejected. He says there's info on Adobe site that confirms he can't upgrade from CS2 (full) to CS4 Photoshop; he has to upgrade from a version of Photoshop.
I'll tell him what you said.