You can open a GarageBand project in Logic Pro just as you would open a Logic Pro project. The GarageBand project translates 1:1 into Logic Pro:
Logic Pro 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 Pro. GarageBand instruments are automatically installed with Logic Pro. Any mixing data and effects (plus their settings) used by software instruments or Apple Loops–based tracks in GarageBand are also imported into Logic Pro.
The channel strip settings of GarageBand translate 1:1 into Logic Pro—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 Pro > Preferences > Audio > I/O Assignments > Output pane.
The two bus effects of GarageBand (Reverb and Echo) are also translated when opened in Logic Pro. They are replaced by the PlatinumVerb and Echo on busses 1 and 2.
Once the GarageBand project is loaded into Logic Pro, you can freely change parts, mixing levels, and plug-in parameters as in any Logic Pro project.
Note: You cannot open Logic Pro projects in GarageBand, nor can you export a Logic Pro project in a format that can be read by GarageBand (except as an audio file).