No, you won’t lose all your DRM-free iTunes music. At least, not without deleting your actual files and not having a backup. Apple isn’t adding DRM to your iTunes files, either.
The reality here is that Apple will not automatically remove any iTunes music files you own on your computer and replace it with a digital rights managed (DRM) file.
However, the convergence of iTunes Match, Apple Music, and the new iCloud Music Library can be confusing, and there is a small potential to re-download files you’ve deleted from your Mac as DRM-protected Apple Music files.
Luckily, the folks at iMore have a pretty fantastic, clear explanation of what’s going down here, and a pretty neat way to check and see which of your music files have been matched, uploaded, or purchased. Even John Gruber linked to it, so you know it’s good.