I phoned Adobe to make the change this evening. The only cost to go from CS3 Design Premium Windows to Mac is shipping for the new box, and as Jay mentioned I have to Fax in my signed Letter of Software Destruction.
I cannot use CS2 on my Windows machine. It has to be destroyed, as my copy of CS3 was an upgrade to CS2, not a full purchase.
With Lightroom, I cannot run the Windows and Mac versions with one license. I have to unactivate and uninstall the Windows version and then install and activate on the Mac. So while two copies of LR can be installed under one license, they have to be on the same OS.
The phone call with Adobe went well. They were helpful and quick. They seem to be quite practiced in doing this.
Richard