You must call Adobe Customer Service to arrange the cross platform switch. They will have you download a letter of destruction (agreeing to destroy your current copy) that you must sign and fax to them. You must own a full version of the software that is registered to you and pay for shipping of the new software.
I did it in the spring. Part of the process was really irritating because the person on the phone checking the serial number didn't ask for the proper one first. He asks for the serial number and I give it to him but he says that is not correct. Then we proceed to go down the bunny trail of every version of Photoshop I ever owned down to Photoshop 5. I have registered every update I have owned so why is this info not in their database? It is a good thing that I still owned the actual copies of each one to get the serial number off the box or he would not have approved it.
