You can open a GarageBand project in Logic Express just as you would open a Logic Express project. The GarageBand project translates 1:1 into Logic Express:
Logic Express automatically creates the required number, and type, of tracks to mirror those used in the GarageBand project.
The project uses the tempo of the GarageBand project.
The initial key signature is set in accordance with the “project key” of the GarageBand project.
Software instrument parts are played by GarageBand instruments in Logic Express. GarageBand instruments are automatically installed with Logic Express. Any mixing data and effects (plus their settings) used by software instruments or Apple Loops-based tracks in GarageBand are also imported into Logic Express.
The Channel Strip settings of GarageBand translate 1:1 into Logic Express—with the nice exception that you can now access the individual plug-ins that are inserted into a GarageBand channel strip.
All channel strip objects are routed to Stereo Out (default stereo output pair), and immediately play out according to the Stereo Output preference chosen in the Logic Express > Preferences > Audio > I/O Assignments > Output pane.
The two bus effects of GarageBand (Reverb and Echo) are also translated when opened in Logic Express. They are replaced by the PlatinumVerb and Echo on busses 1 and 2.
Once the GarageBand project is loaded into Logic Express, you can freely change parts, mixing levels, instrument, and effect parameters as in any Logic Express project.
Note: You cannot open Logic Express projects in GarageBand, nor can you export a Logic Express project in a format that can be read by GarageBand (except as an audio file).