EF shows the following fare rules for a Z fare out of YYC to LHR:

"CANCELLATIONS

ANY TIME
CHARGE CAD 500.00 FOR CANCEL/NO-SHOW/REFUND.
CHILD/INFANT DISCOUNTS APPLY.
WAIVED FOR DEATH OF PASSENGER OR FAMILY MEMBER.

CHANGES

BEFORE DEPARTURE
CHARGE CAD 400.00 FOR REISSUE/REVALIDATION.
CHILD/INFANT DISCOUNTS APPLY.
WAIVED FOR DEATH OF PASSENGER OR FAMILY MEMBER.
NOTE - TEXT BELOW NOT VALIDATED FOR AUTOPRICING.
BEFORE DEPARTURE OF JOURNEY - FOR O/B CHANGE
THE NEW BASE FARE AMOUNT MUST BE
RECALCULATED USING CURRENT AND APPLICABLE FARES IN
EFFECT AT THE DATE OF ISSUANCE. CHARGE DIFFERENCE
IN FARE PLUS CHANGE FEE. CHANGE FEE IS SUBJECT TO
EACH CHANGE TRANSACTION.
------------------------------------------------
BEFORE DEPARTURE OF JOURNEY - FOR I/B CHANGE
THE NEW BASE FARE AMOUNT MUST BE RECALCULATED
USING HISTORICAL AND APPLICABLE FARES IN EFFECT
AT THE ORIGINAL TICKET DATE. CHARGE DIFFERENCE
IN FARE PLUS CHANGE FEE. CHANGE FEE IS SUBJECT TO
EACH CHANGE TRANSACTION.
-------------------------------------------------
NEW FARE AMOUNT MAY BE LOWER/EQUAL OR HIGHER THAN
ORIGINAL FARE. IF NEW FARE IS LOWER THAN ORIGINAL
FARE-NO REFUND PERMITTED. THE RESIDUAL AMOUNT IS
FORFEIT."

Looks to me like difference in fare will be charged plus change fee.