Aperture 2 library "cannot be upgraded" to work with Aperture 3; why not?

Just bought and installed Aperture 3,latest version. Before loading new program, I unplugged external hard drive where my Aperture library is stored. Just wanted to make sure nothing happened to the content, in excess of 60,000 images. After booting new Aperture, tried to open a smaller library resident on compeuter. Followed instructions on initial window to upgrade library, but got the message, library "cannot be upgraded". Getting irritated because I need to complete some work in progress on an existing project. I know, I should have done that before upgrading Aperture, but I had no idea that I would run into this. Any help?