Bad experience

I bought take a friend for free ticket for myself and family which is total of 4 from Mel to BKK direct flight. It cost s me $1700 includes 20 kg luggage and seat selection. Then I got mail that I have been re schedule so I need to stopover in Singapore on the who to BKK for 7 hours. I called call centre they said can change to earlier flight but I have to pay $1700 for 4 passenger because 2 of us is free ticket so need to purchase new ticket. Then she recommend that if I leave this ticket And buy another one from SIN to BKK. It will cost me $95 SG dollar per passenger. And I need to get out at SG airport and re checking in again. So what's the hell? They're so cheating.

International
Australia-Melbourne - Tullamarine
Thailand-Bangkok