You should be able to cross grade. The procedure involves sending a letter of destruction of your PC copy. Contact Adobe customer service for more info.
You can, but you need to contact adobe customer support, you will need (not only a registered PS 5.5 serial) to pay for both an upgrade, and a cross-grade fee. In the past the cross platform fee isnt really that much, really the important factor is having the PS 5.5 registered serial.
Again, only cust support can fully answer your question.
If you’re going to upgrade, I strongly suggest you do it now.
Adobe is changing their upgrade policy and only PS 7.0 and later will be eligible for upgrades to CS3.
Bob
Really, changing upgrade policy? Good to know, as always.