Call Delta and see if they can change your flight to something else. I'm not sure from your post if the new departure time is 3:45 pm instead of 6:40 am, but if it is you should be eligible to switch your flight at no additional cost.
What was your original arrival time? It must be a short, direct flight if you plan to fly out at 6:40 am and arrive in Orlando in time to take the Magical Express, check into your hotel and make it to Ohana by 11 am.
I think (after reading the OP a couple of times) that this is the flight home and it was originally at 6:40pm but moved up to 3:45pm. Since DME would pick them up around 12:45pm (instead of around 3:40pm), the 'Ohana reservation is now too late (the OP made a comment about looking for an earlier reservation).
OP - Did you actually look at your flight booking to confirm that it's been changed and there hasn't just been a change in flight numbers? Weirdly, for example, it appears that Delta used DL2517 for both a DTW to MCO flight and the MCO to DTW flight.
Once you've confirmed that your flight has changed, as PP have said, you have options:
1) Look at the Delta website and see if there is a flight that works better for you than the 3:45pm flight. This may involve switching from non-stop to direct or connecting, but that may be worth it to you. Given the amount of time your flight was changed, you should be able to do this for no charge (even if the new flight is more expensive - but not if you'd have to go up a class (i.e. only First class left on the flight).
2) Try to move up your 'Ohaha reservation.
3) Take a taxi/uber directly from 'Ohana (I'm not sure if breakfast is better than dinner, but dinner often has really delayed seating and slow service, so I personally would worry with this option (but I have a low tolerance for timing risk - probably fine for most people)).