Got a discounted ticket for SIN BKK SIN some time back. On the return sector, I was thinking of changing the flight to an earlier one. Called SQ Res in BKK, and they say that my ticket is non-changeable, i.e. that I must fly on the flight stipulated in the ticket.

Went to the airport earlier anyway cos there wasnt much thing to do at the hotel. They are doing the check in for the earlier flight. I went to the desk, checked in, and was offered a seat in the earlier flight by the check-in agent. He simply asked, "I have seats in an earlier flight. Would you like to take it?".

I was "Yeah, sure!". And there I was, I got a seat.

Yeah, I think the check-in agents can override the restrictions.