I often get asked about the upgrades for Photoshop and this is a very tricky question.
First of all, you can only upgrade three versions back. If you have Photoshop CS2, you can upgrade to CS5. If you have CS, you cannot. Sorry.
You also need to be careful where you buy it from. There were a number of threads on Adobe Forums on this topic and we don’t encourage you to do so. It’s just too risky.
Here is the link to the thread, if you are interested:
Thinking of buying from eBay? – Adobe Forums.
I would also be very careful when buying a “used” upgrade. Remember that Upgrades are tied to the original license and cannot be transfered so there are no “used upgrades”.
Good luck and remember – be careful. 😉
Have a great day!