Songs (Pre owned MP3's etc) you have stored locally and songs you have on OneDrive will continue to play as before, the ones you have added to your collection via groove should revert to samples only so as far as I am aware you will get a 30 second trail (Or maybe no trail or play) of them just like ones you haven't added to your collection, and again as far as I am aware this will apply to any you have added to your collection and downloaded.
The ones you downloaded I think will stay on the device (In case you change your mind and go back to a pass).
Bottom line is, if you already owned it you can play it, if you effectively leased it via Groove music pass, you wont.