When you update to macOS 10.15 and the new Music app, your current iTunes library will be imported just as if you were simply updating to a new version of iTunes. You can also open an iTunes Library.itl file as if it were a "multiple library" by press-and-holding Option as you launch Music to get the "Choose
iTunes Music Library" option.
I first saw this at the Apple Developer Forums. I tried it and it worked great. I haven't been using my main library while Catalina is still in beta because, well, it's my main library and Catalina is still in beta. But this was an easy way to have it available as on-demand optional library.
I copied the entire contents of ~/Music/iTunes/ to a new folder, except for my audio files which are stored elsewhere on an external drive; quit Music and restarted while holding the Option key. The dialog pops up asking you to "Choose Music Library" and I chose the "iTunes Library.itl" file from the folder I just copied the stuff to. Music then asks you to name and save the library. In a few minutes, the library was imported. The old "iTunes Library.itl" I selected was copied as a new new-style "Music Library.musiclibrary" and everything looked as I expected when the import was finished.