Official comment
To transfer the rekordbox library, we recommend using the File > Library > Backup Library function. During the backup procedure, one of the steps asks if you want to backup the music as well - you can select yes, then it will also migrate all your music into the backup. Before you leave the old computer, go to Preferences > License and click the Deactivate button to release your license from that computer.
Then on the new computer, during the rekordbox installation, you can activate that copy with your license, then use File > Library > Restore Library to bring all your data back in.