I just bought a new computer and decided to start fresh, rather than migrate my old system over (I was having some issues and it seemed like an ideal time to do this).
My old computer was purchased used, and came with Lightroom 3.x installed. I bought an upgrade to LR 4 for $79 right when it came out.
I went into my Adobe account and downloaded LR 4. When I put the serial number in, it told me that it's an upgrade serial (which is correct), and I need to enter the old serial. Unfortunately, I don't have it written down and it's not in my Adobe account, because LR 3.x came installed on the computer I bought.
What can I do? Is there a way to retrieve the serial from LR on the old computer?
If you go to LR3 Help menu on the old computer > System Info, it should list the serial number. That said, the old registration is likely registered to the previous owner (and still used by them?), so I should just mention that upgrading from the license may not be legal under the terms of the license agreement.
Probably easier to return the upgrade and buy the full version, if you need to do that. Adobe are pretty good about refunds http://helpx.adobe.com/x-productkb/policy-pricing/return-cancel-exchan ge-order-north-america.html
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