Make a backup copy of your Aperture 2 library, if you do not have one. Then open the Aperture 2 library in Aperture 3.5 by double clicking it and holding down the ⌘⌥ keys at the same time. Let Aperture repair the database before upgrading it to Aperture 3 format.
After that you can use Aperture 3 to export a partial library by selecting the projects you want to export and using "File > Export > Project as library". Then import the partial library into your new Aperture library:
- Double-click the new library to switch to it.
- Use "File > Import > Library" to import the small library with your selection of projects.