Here's what Adobe say:

"Am I eligible for upgrade pricing from an earlier suite edition to a standalone product?

No. Upgrade pricing is available only if you have one of the qualifying earlier products listed on the upgrade eligibility page of the product you wish to purchase and can provide the serial number for that product during installation.

Creative Suite Premium or Standard; Creative Suite 2 Premium or Standard; Production Studio; and Macromedia Studio do not qualify for upgrade pricing on standalone products such as Photoshop CS4, Illustrator CS4, or Dreamweaver CS4."

So it can't be done.
The only other version I have of Photoshop is Photoshop 7; but that's not in their upgrade list either.
I shall continue with my CS2 version.