Thanks for the quick responses. The initial ticket was purchased on the morning of Oct. 28 for a same day departure. Looking back at the actual email receipts, Google timeline, etc., I now remember that I actually made the change from the SkyClub while waiting for my outbound departure, engaged in conversation with a colleague. Also, I did agree to the change while not listening closely... just shocked by the fare, because I first tried to do it myself, then started making a dummy booking for the flight I wanted to check the price (which was a few hundred $ cheaper) before calling.

The original itinerary booked before I left for the airport was:
28 Oct DL1770 DFW LGA Fare basis: HA0UA0FB (booked into F cabin, under D class)
30 Oct DL548 LGA DFW Fare basis: HA0UA0FB (booked into F cabin, under D class)
Fare Details: DFW DL NYC546.98HA0UA0FB DL DFW546.98HA0UA0FB USD1093.96END ZP DFWLGA XF DFW4.5LGA4.5
Total fare: $1,204.60

Following the change made on the phone while at the SkyClub prior to departure resulted in the following:
28 Oct DL1770 DFW LGA Fare basis: HA0UA0FB (booked into F cabin, under D class)
31 Oct DL1038 LGA DFW Fare basis: YA0RA0FL (booked into F cabin, under J class)
Fare Details: DFW DL NYC546.98HA0UA0FB DL DFW1008.37YA0RA0FL USD1555.35END ZP DFWLGA XF DFW4.5LGA4.5
Total fare: $1,700.60

Any thoughts? I know this was primarily my fault for not paying close attention to the phone agent, etc.