Depends on the fare rules. However, typically fare rules may include a change fee, and also state:
Code:
CALCULATION FOR REISSUES
// CHANGES BEFORE OUTBOUND DEPARTURE //
WHEN THE FIRST FARE COMPONENT IS CHANGED THE
ITINERARY MUST BE REPRICED USING CURRENT FARES
IN EFFECT ON THE DATE THE TICKET IS REISSUED.
---
WHEN THERE ARE NO CHANGES TO THE FIRST FARE
COMPONENT BUT OTHER FARE COMPONENTS ARE CHANGED
THE ITINERARY MUST BE REPRICED USING HISTORICAL
FARES IN EFFECT ON THE PREVIOUS TICKETING DATE
OR CURRENT FARES IN EFFECT ON THE DATE OF TICKET
REISSUE WHICHEVER IS LOWER.
---
// CHANGES AFTER DEPARTURE //
THE ITINERARY MUST BE REPRICED USING HISTORICAL
FARES IN EFFECT ON THE PREVIOUS TICKETING DATE.
THE NEW ITINERARY MUST MEET ALL RULE PROVISIONS
OF THE NEWLY TICKETED FARE -I.E. ADVANCE
RESERVATIONS/TICKETING DEADLINE/MINIMUM/MAXIMUM
STAY/BOOKING CLASS/SEASONALITY/ETC-.
DOWNGRADING IS NOT PERMITTED FOR ECONOMY FARES.
THE NEW FARE MAY ONLY BE EQUAL OR HIGHER THAN
PREVIOUS. ANY CHANGE WITHIN THE SAME TYPE OF FARE
INVOLVING SEASONALITY OR DAY/TIME IS NOT
CONSIDERED DOWNGRADE.
Changes after departure may therefore be cheaper since the reference is to historical fares. In both cases the change fee is payable though.
However, frankly based on the low cost you have been quoted to change (all or a large part of which may well be unavoidable change fee) I would just do it now for the sake of certainty. I doubt there is much fare difference being included atm.