I had my kids booked on travel in the T class (international flight, VCE to JFK) and the U class (domestic JFK to BUF) and I had to change the return to a couple of days earlier. I was charged $ 20 each ticket, only for the assistance of the telephone ticket agent. The return U class was rebooked to an L class fare.

I was told it would have cost $ 250 but the fare ended up being slightly lower on the date I made the change so they only charged $ 20. This is not something I can count on, I suppose.

FWIW, these tickets would be for the same route, VCE-JFK-BUF in June returning September. Guess I'll be doing some digging.