Checking an S to Chicago the rules are as follows :-
ANY TIME CHARGE GBP 90.00. WAIVED FOR ILLNESS OR DEATH OF PASSENGER OR FAMILY MEMBER. NOTE - TEXT BELOW NOT VALIDATED FOR AUTOPRICING. BEFORE DEPARTURE OF THE OUTBOUND JOURNEY FOR CHANGES MADE PRIOR TO THE DEPARTURE OF THE SCHEDULED FLIGHT - REPRICE USING CURRENT FARES.
One other small thing, there will still need to be available S class fares for the flight you want to switch too, if there's only W available you'd then need to also pay the fare difference as well as the change fee!