Steps of this incident. original reservation was MCO-ATL-DCA
1. Arrived at MCO for 1730 flight at 1400. Shortly after arrival the flight was cancelled due to weather.
2. Called Skymiles member line immediately and explained it was very important we get to DC. Agent searched for alternate flights but none were available. I offered to drive to ATL and fly from there. We already had seat assignments
3. Agent said I will issue a ticket for that segment. Sorry about the MCO flight cancellation.
4. Immediately received a confirmation email. Also, checked my Skymiles app and the website and noted new boarding passes OU [complementary upgrade] seat 3A and 3B.
5. Arrived at ATL several hours before flight. Went to Sky Priority check in to check my bag. Was issued boarding passes for 3A and 3B.
6. Was told by gate agent I didn't have seat and may not get one.
History shows on Delta's system I had seats one hour before boarding started according to Delta representative who investigated later.
I have the boarding passes.