Databases created with release 1.5.x need to be exported and reimported in OrientDB 1.6.x.
From OrientDB 1.5.x:
./console.sh (or .bat on Windows)
./console.sh (or .bat on Windows)
For any problem on import, look at Import Troubleshooting.
OrientDB 1.6.x comes with the new PLOCAL engine. To migrate a database create with the old "local" to such engine follow the steps in: Migrate from local storage engine to plocal.