Can I keep my iTunes library after I get an Apple Music subscription?
You can keep your iTunes music.  A subscription to Apple Music just provides you access to most of the music found in Apple's iTunes music store.  It will not replace any of the songs that are found in your library.  It is important to note that all of my songs were downloaded from the store itself.

If you are worried about your songs being deleted or overwritten, I would suggest that you back them up to an external HD or another folder on your computer.  Then if they are overwritten you can always get them back.

One thing to note though is that on all computers/iPhones/etc. besides the one you initially subscribed to Apple Music, any music downloaded as part of the subscription will come from Apple Music. In most cases this will be the same track as the one on the original computer, but if Apple's matching algorithm gets it wrong, you might get another version of the same track (e.g. live vs studio recording, or original releases vs. "best of" sampler).