Will Signing up for Apple Music mess up my itunes match library?

As long as you have a primary library which retains local copies of all your files, and a backup of the same, then it is all good. I switched up from iTunes Match to Apple Music when HomePod came out so I could have a free choice of music to call on. See Backup your iTunes for Windows library with SyncToy - Apple Community for my suggested backup strategy.

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Why don't you need a backup? The matching process isn't guaranteed to be perfect. If, for whatever reason, you lose local copies of your files and only then discover that the version held in the cloud isn't quite what you're expecting then that is where a backup comes in. When you delete from iTunes Match/Apple Music that deletion is replicated everywhere. On iOS that is immediate, for locally stored tracks in an iTunes library tracks are marked as removed rather than eliminated from the library, but I seen posts where users thought they were freeing up space on their computer only to find they'd irrevocably thrown content away. Apple Music shouldn't alter any content in your library, although I have seen both services reset metadata for purchased content. When this has happened to me I've hidden those items from my purchase history so that my preferred version is retained. I'm very particular about Title Case for example.

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