DL adds a specific surcharge for domestic F fares on D1 equipment (up to $150 for a 744). So I suppose you could request a refund in the case of an equipment swap.

A SURCHARGE OF USD 100.00 PER FARE COMPONENT WILL BE ADDED
TO THE APPLICABLE FARE FOR TRAVEL IN AIRBUS INDUSTRIE
A330-300 EQUIPMENT.
AND - A SURCHARGE OF USD 100.00 PER FARE COMPONENT WILL BE
ADDED TO THE APPLICABLE FARE FOR TRAVEL IN BOEING
777-200LR EQUIPMENT.
AND - A SURCHARGE OF USD 100.00 PER FARE COMPONENT WILL BE
ADDED TO THE APPLICABLE FARE FOR TRAVEL IN BOEING
767-400 EQUIPMENT.
AND - A SURCHARGE OF USD 100.00 PER FARE COMPONENT WILL BE
ADDED TO THE APPLICABLE FARE FOR TRAVEL IN BOEING
777 EQUIPMENT.
AND - A SURCHARGE OF USD 100.00 PER FARE COMPONENT WILL BE
ADDED TO THE APPLICABLE FARE FOR TRAVEL IN BOEING
767-300 WINGLETS EQUIPMENT.
AND - A SURCHARGE OF USD 150.00 PER FARE COMPONENT WILL BE
ADDED TO THE APPLICABLE FARE FOR TRAVEL IN BOEING
747-400 PASSENGER EQUIPMENT.
AND - A SURCHARGE OF USD 75.00 PER FARE COMPONENT WILL BE
ADDED TO THE APPLICABLE FARE FOR TRAVEL IN BOEING
757-200 WINGLETS EQUIPMENT.