When you book a flight, with any airline, they are only required to get you from point A to point B on a particular day. Read the fine print..it's incredible. Yes, Delta can, and frequently does, change flights. And when you book more than a month or so before flying, you stand a huge chance of many changes. I booked my flight to MCO in Sept about a month or so ago. I have already had changes to the flight and last week they completely canceled my flight and put me on an earlier one. Delta has made announcements that they are cutting back routes to Orlando for about a week now, so I wasn't surprised to see my non-stop flight canceled.
They will not notify you until just before your flight (if at all!!). The traveler should take it upon themself to check on their flights on a regular basis to see what changes have happened. When I see a change to my flights, there is usually a seat change as well. I then call Delta and have my seats reassigned to where I want to sit. I have had to call a total of 13 times for one flight about two years ago!! It's a hobby for me now!