Does Delta ever put bags on an earlier plane?
Yes. They may even reroute your bags on a more direct routing. I had DTW-SEA-LAS and while I was unable to change to the nonstop, my checked bag was put on a DTW-LAS nonstop and was waiting for me at LAS when I got there (which I actually wasn't overly happy about because it was just sitting in a large area with several other bags waiting to be claimed and theoretically anyone could have just come by and picked it up and walked out with it).