According to what I just read
HERE, you will be able to use your CS2 serial number with the CS3 version.
Unless I misundersood, you won't have to repurchase the program when you upgrade. When I went from PS7 to CS2 I just installed the program in CS2 and it automatically picked up the number from the PS7 installation and ran. No muss, no fuss.
If I am wrong I am sure Jeff will correct me
Rich