Classic change fee is 3500 afaik.
Thats the only fee payable if its in J or F, if its Y then a classic attracts the 8k service fee as well, which means your better of cancelling for 5K
Change Fee
For Classic Award changes: 3,500 Points per passenger.
Where changes are made to Classic Awards via Qantas Telephone Sales, Service Fees (below) are payable in addition to the Change Fee.
Service Fees
Service Fees are payable in addition to any applicable Change Fees.
Payable where changes are made to Qantas & Partner Classic Award bookings via Qantas Telephone Sales.
Money payment option is available for Award Flight booking changes made in Australia or in New Zealand (except as otherwise stated). Amount must be paid at the time of making your changes.
Service Fees will not be applied when changing Classic Awards booked for travel in Business or First class.
For Domestic AU / NZ: 4,000 Points per passenger
For Trans Tasman: 4,000 Points per passenger
For International: 8,000 Points per passenger
OR (where money payment option available – money payment option is not available for changes made to NZ Domestic bookings): The A$ or NZ$ change fee set out in the 'Fees for making changes to Bookings' table at qantas.com
Cancellation Refund Fee 5,000 Points per passenger
FWIW I would have a look at options to go HKG SIN as a paid fare and enjoy the good equipment running to HKG (assuming its not ex BNE)