You can open Logic 9 projects in Logic 8 without conversion. Logic 8 will display a warning the project was created in a newer version.
However be aware you will not be able to use any of the new Logic 9 features (e.g. Flex and new plug-ins) in Logic 8, and therefore may get unexpected results when opening a Logic 9 project in Logic 8.
Thanks for replies so far. One of my students has Logic 9, the institution I teach at still has 8. He told my colleague the other day that he couldn't get it to work; I'll should see him today and be able to check it out for myself.
Based on my experience today, a Logic 9 file can be opened in Logic 8.
The project was an all MIDI tune, started on my Mac Pro using only Logic software instruments. A warning message about the project being created in a later version of Logic popped up in Logic 8 when opening the file.
But it opened intact (on a dp G4 running OSX 104.9) with all assigned instruments/presets.