I´ve done same-day changes at the airport to an earlier flight or an alternative routing several times and am familiar with the procedure, however I´ve never tried to do a same-day change to a later flight. is this possible?

For instance, if I´m booked on the day-time flight YYZ-LHR, could I show up at the airport at 6a.m. and ask to be rebooked on the 10p.m. flight to LHR (assuming there´s availability, of course)?

Thanks!