Adobe won't charge you anything.
When I switched to Macs they had me sign a "Software destruction Agreement" and mail that to them. Once they received that contract, they sent me a full copy of CS2 for the mac.
Call their customer support and they'll give you a case number and tell you the url to download the agreement (pdf).
Thatís if you agree to destroy the original version but if you want to use both, youíre going to pay. Thats fair. They donít have to give you a discount on a side grade (I suspect some companies donít).
I will check to see if Pixel Genius has a similar agreement as Adobe whereby you can pledge to destroy the original version and get a free side grade. I know they offer a discount on the 2nd OS but the assumption is, youíll use both.
The major difference with Adobe is the activation. I suspect if you agree to toss the original OS (in writing), they can still ensure you canít activate it after that. Not sure if itís already activated they can undo that however.