iTunes is going away but don’t worry. Apple Music has you covered, including the ability to automatically transfer your music files from your iTunes library to your Apple Music library.
Jessica Dolcourt at CNET has an excellent Q&A post that covers everything you need to know, including “Every song you’ve ever bought, ripped, uploaded or imported will already be part of Apple Music when you upgrade from your current Mac OS version to Catalina. All the files that are already on your computer will remain. Apple isn’t liquidating anything you already own, but it will reorganize where the files live.”