Apple Music vs. personal music collection

I have an extensive music library full of tracks that I have purchased prior to the creation of subscription based Apple Music.

That is fine, and it is yours to keep. You should also have a backup, though. Apple Music isn’t able to put everything back as it was before, with edits and all, if you cancel one day.

They also say access to downloaded content will not be available if the Apple Music subscription is cancelled.

That is only true for the subscription based content, not the prior music/purchased music. The purchased content (iTunes Store) and audio from other sources than the subscription should stay as-is. You may get offered an opportunity to replace these other tracks with iCloud versions, but I would advise to keep the iTunes ‘master’ library with the non-DRM ‘originals’.

I have songs that I have bought and paid for in my iTunes library that have the iCloud download symbol. I have automatic downloads enabled.

Content purchased in the iTunes Store can be ready-to-download, and can have such an icon.

Can Apple Music control the tracks I purchased in my iTunes library?

The new Library will be a mix of pre-existing content and subscription content.

Access your music collection on all of your devices with Apple Music - Apple Support

• What happens to the music you already had on your devices when you subscribe to Apple Music?
• If you have music on your iOS device or computer from sources other than Apple Music or the iTunes Store