In the U.S., at least, sales receipts do not seem to be necessary. A friend just switched from PC to Mac, and this is what he told me:
"You have to call Sales at Adobe. (1-800-833-6697). They look you up in their system and then make you download a form that you fill out and fax back to them. (It has all the info they request (make sure you give them your latest version serial number as well as version 7 serial number (they want a full version serial along with your upgrade serial) plus a statement saying you will destroy your existing versions of the software.
"They didn't tell me they needed the second serial number until I called them yesterday to see why I hadn't got mine yet. They only charge for shipping (unless you don't already have CS2 then they'll charge you for an upgrade as well). I think they told me that you are limited to 5 or 7 lifetime cross platform switches."
and now somebody explain to me why somebody with legal license to cs should not use crack but spend time calling etc.