Unclear as the downgrade occurred on a USA domestic segment, although the downgrade was the result of the delay on the "TATL" segment on the same PNR/ticket. If it applies, the downgrade compensation would be 75% of the fare applicable to that segment only. In this case, to further complicate matters, the rebooking added a connection, so that the downgrade was on a segment that was shorter than what was originally booked. DL might argue that the OP had the option of waiting longer for a FC only routing.

Unfortunately, the fare difference refund method of calculating downgrade "compensation" rarely results in significant amounts of money as broken fares tend to be more expensive.