Changing Venice–Bangkok to Phuket, how much extra?

Booked an Economy Classic T fare through an OTA (Venice to Bangkok) and thinking about switching the outbound to Phuket while keeping the return the same. Qatar support said I can do it directly with them within 4 days of departure. Current fare difference is about $400 total for 4 people. Anyone know what the change fee usually is on this fare type, or if it’s basically $400 plus the fee?