I do think DL (or their agents) misapplied the fare rules. Whether or not that would have made a major difference in the amount you needed to pay is difficult to determine without more information.
The reissue rules of a typical international restricted coach fare JFK-IST are (probably very similar to your fare):
Code:
TICKET REISSUE PROCEDURES
1. CHANGES TO OUTBOUND PORTION OF UNUSED TICKETS
A. ISSUE A NEW TICKET - CANCEL AND START OVER
B. MUST USE CURRENT FARES AND IROE WHERE APPL
C. VERIFY ALL RULES
D. VALUE OF THE ORIGINAL TICKET LESS THE
APPLICABLE CHANGE FEE MAY BE APPLIED
TOWARDS THE PURCHASE OF A NEW TICKET.
2. CHANGES TO CONTINUING/RETURN OF UNUSED TICKETS
A. ATTEMPT TO KEEP THE FARES AS TICKETED
-CANNOT CHANGE ORIGIN/DESTINATION OF ANY FARE
COMPONENT
-MUST BE ABLE TO KEEP THE ORIGINAL FARES AS
TICKETED FOR ALL FARE COMPONENTS
-USE HISTORICAL FARES IN EFFECT ON THE DATE
OF THE ORIGINAL TICKET ISSUE
-ALL OTHER RULE DATA MUST BE VERIFIED
B. REPRICE KEEPING OUTBOUND FARES AND REPLACING
THE CONTINUING/RETURN FARE COMPONENTS USING
HISTORICAL FARES AND MODIFIED ADVANCE
PURCHASE
-CANNOT CHANGE ORIGIN/DESTINATION OF THE
FIRST FARE COMPONENT
-USE HISTORICAL FARES IN EFFECT ON THE DATE
OF ORIGINAL TICKET ISSUE
-REPLACEMENT OF FARES MUST BE EQUAL OR HIGHER
FARE TYPE
-MODIFIED ADVANCE PURCHASE
DETERMINE THE APPLICABLE ADVANCE PURCHASE BY
CALCULATING THE NUMBER OF DAYS BETWEEN THE
ORIGINAL TICKET ISSUE DATE AND THE DATE OF
DEPARTURE OF THE FIRST FLIGHT SEGMENT. THE
ADVANCE PURCHASE OF ANY REPLACEMENT FARE
MUST BE EQUAL TO OR LESS THAN THIS NUMBER
-ALL OTHER RULE DATA MUST BE VERIFIED
INCLUDING FARE COMBINABILITY
C. ISSUE NEW TICKET - CANCEL AND START OVER
-MUST USE CURRENT FARES //VERIFY ALL RULES
-MUST BE HIGHER FARE TYPE
-VALUE OF ORIGINAL TICKET LESS APPLICABLE
CHANGE FEE MAY BE APPLIED TOWARDS THE
PURCHASE OF A NEW TICKET.
3. CHANGES TO PARTIALLY USED TICKETS
A. ATTEMPT TO KEEP THE FARES AS TICKETED
-CANNOT CHANGE ORIGIN/DESTINATION OF ANY FARE
COMPONENT
-USE HISTORICAL FARES IN EFFECT ON THE DATE OF
THE ORIGINAL TICKET ISSUE
-ALL OTHER RULE DATA MUST BE VERIFIED
INCLUDING FARE COMBINABILITY
B. REPRICE KEEPING FARES FOR FLOWN FARE
COMPONENTS AND REPLACE FARES FOR UNFLOWN FARE
COMPONENTS USING HISTORICAL FARES.
-MAY CHANGE ORIGIN/DESTINATION ON ANY FARE
COMPONENT
-USE HISTORICAL FARES IN EFFECT ON THE DATE
OF THE ORIGINAL TICKET ISSUE AND MODIFIED
ADVANCED PURCHASE
-ALL OTHER RULE DATA MUST BE VERIFIED
INCLUDING FARE COMBINABILITY
-REPLACEMENT FARES FOR UNFLOWN FARE COMPONENTS
MUST BE OF EQUAL OR HIGHER VALUE/BOOKING
CLASS AND FARE TYPE THAN THE ORIGINAL
TICKETED FARE
-NEW TICKET VALUE MUST BE OF EQUAL OR HIGHER
VALUE THAN THE TICKETED FARE
-USE BOOKING CLASS HIERARCHY BELOW WHEN
REPLACING FARES FOR UNFLOWN FARE COMPONENTS
ECONOMY CLASS - Y B M H Q K L U T
BUSINESS CLASS - J C D S I
FIRST CLASS - F A
C. REPLACE FARES FOR FLOWN AND UNFLOWN FARE
COMPONENTS WITH HISTORICAL FARES.
-MAY CHANGE ORIGIN/DESTINATION OF ANY
FARE COMPONENT
-USE HISTORICAL FARES IN EFFECT ON THE DATE OF
THE ORIGINAL TICKET ISSUE
-ALL THE OTHER RULE DATA MUST BE VERIFIED
INCLUDING FARE COMBINABLILITY
-REPLACEMENT FARES FOR FLOWN AND UNFLOWN FARE
COMPONENTS MUST BE OF EQUAL OR HIGHER
VALUE/BOOKING CLASS AND FARE TYPE THAN THE
ORIGINAL TICKETED FARE
-NEW TICKET VALUE MUST BE OF EQUAL OR HIGHER
VALUE THAN THE TICKETED FARE
-USE BOOKING CLASS HIERARCHY BELOW WHEN
REPLACING FARES FOR FLOWN AND UNFLOWN FARE
COMPONENTS
ECONOMY CLASS - Y B M H Q K L U T
BUSINESS CLASS - J C D S I
FIRST CLASS - F A
D. ISSUE A NEW TICKET
-MAY USE VALUE OF ORIGINAL TICKET LESS CHANGE
FEE TOWARD THE PURCHASE OF A NEW TICKET
-THE ORIGINAL NON-REFUNDABLE AMOUNT REMAINS
NON-REFUNDABLE AND MUST BE NOTED IN THE
ENDORSEMENT BOX
-VALIDATE ALL RULES
-IF ANY RESIDUAL VALUE IN THE ORIGINAL TICKET
/LESS THE CHANGE FEE/ IT MAY BE APPLIED
TOWARDS PURCHASE OF NEW TICKET
E. FARE TYPE HEIRARCHY
A FARE TYPE MAY BE UPGRADED TO ANOTHER FARE
TYPE ON A LEVEL UP FROM ITS OWN. FARE TYPES
AT THE SAME LEVEL MAY NOT BE USED.
EXAMPLE...AN XAC MAY BE UPGRADED TO AN XAB
OR ABOVE. YOU MAY NOT USE AN XPY AS IT IS
ON SAME LEVEL AS THE XAC
.
FR/FU
FX/FOX
BR/BU
BX/BOX
BAP
BXN
ER/EU
XEX
The rules specify that the flown segments should *not* be repriced and that agents are to use historical fares for the available booking class for the new flight.
The difference could very well have been less than $900.
The change fee, though, is usually $250.
If I were in your shoes I would file a complaint with the DOT (link in my signature). The DOT won't take action based on one complaint, but it will ensure that a manager at DL will review your complaint.
Be very specific--that you believe DL misapplied the fare rules with respect to refaring the return portion of a roundtrip ticket, quoting you ca. $1400 (based on repricing the entire ticket, including the portion already flown) and thus making it more economical for you to purchase a one-way ticket.
In the future, I would suggest that you might need to escalate at the airport and try to call DL reservations on the phone.