DL (and all airlines) will change flight numbers for all kinds of reasons. Usually it's just the result of loading a new schedule where they wanted to realign all flight numbers for a certain route or something; and perhaps your flights were one of those realigned or previously held the flight number that is now assigned to a different flight.
At the end of the day, it shouldn't have any impact on you. I would strongly advise you check your itinerary on the DL website or app to confirm that your booking has properly updated to the new flight numbers and you've retained any seat assignments (if that's important to you). The new flight numbers are what will matter on the day of travel.