I changed my son's "grab-a-seat flexitime" just after midnight online for the day he was flying back. Worked well. The only cheap flights for last weekend from CHC to WLG were horrible times [late Saturday, sparrow f*rt Sunday] so we chose the flexitime option [$79 each way], then changed each sector on the day to be much nicer times so as to make the most of the weekend 10 am Saturday, 7 pm Sunday]. There is an element of risk in such an approach as it relies on getting an available seat on the service that works better for you.