Downgrading Aperture 3.4.3 library to 3.0
Attempting to open the library repair tools by holding down ⌘⌥ while opening that library and then clicking Cancel results in Aperture correctly opening it and adjusting the version number in the library file so that it can be normally opened on future attempts.

Adjustments, including the updated Highlights &amp; Shadows, seem to properly downgrade.

Video available at http://youtu.be/JYKSSJ4CQrQ