The 'free' flight is not flexi. It is a Red-E deal or equivalent. Once when I went to book for a particular date and flight, I had to pay some $$ as the fare bucket they use was not available.
I have recently made a booking. Here are the Ts&Cs from that:
Ultimate Qantas Fare Rules
Minimum Stay: None
Maximum Stay: One year from date of issue.
Reservations: Instant purchase fare
Changes: Prior to day of departure
A. Permitted at an airline charge of AUD77.00 including GST per person. Rebooking/Reissue must occur no later than 24 hours after the change is made. Not permitted to
change and travel on the same day.
B. A service fee of AUD27.00 per person will be charged by American Express for all changes.
C. Fare may be used as a credit towards payment of an equal or higher ticket value provided conditions of the new fare have been met. Any higher fare difference will be paid
by card member in addition to the service and reissue fees.
On day of departure Changes not permitted. Fare is forfeited.
Ticket Validity: 1 Year. All travel must be completed within 1 year of original ticketing date.
Ticketing: An electronic ticket must be issued
Restrictions: Non Refundable / Non Endorsable / No name changes permitted
Cancellations: Failure to check in on time for your confirmed flight will result in your ticket being void. Cancellation on day of departure is not permitted. Fare is forfeited.
Frequent Flyer: Frequent flyer points will not be earned on this ticket.
Please refer to www.americanexpress.com.au/ultimate for the American Express flight offer conditions.
If you have any enquiries regarding this confirmed booking or wish to make any changes please contact Platinum Priority Service on 1800 507 104 between 9am to 6pm EST
Monday to Friday.
We hope you enjoy your American Express Ultimate Card return Flight. We wish you a pleasant journey.
Hope that helps.