Iam assuming here that it is all 1 PNR and that the SIN-HKT leg was with deathstar... if not, disregard the following and I'm sure there will be another option.
There may be another option provided under QFs Commercial Policy Statement re QF77/78 issued 13 March ... you may have to plead your case for the change from SIN to HKT,
This may have been posted elsewhere in full, so apologies if it has
Passengers beyond destination Singapore affected by the cancellation of
QF77/78 may, without fee:
Reroute/Rebook
- Travel via Sydney/Brisbane/Melbourne gateway on alternative Qantas service
with 'QF' flight number operated by Qantas.
- Travel to/from Asia/India and South Asian Subcontinent on to alternate
Malaysia Airlines or Cathay Pacific service with ‘MH’ or ‘CX’ flight number
operated by oneworld partners Malaysia Airlines and Cathay Pacific.
- Travel to/from United Kingdom/Europe/Middle East/India and South Asian
Subcontinent on to alternate Qantas codeshare service with ‘QF’ flight number
operated by Emirates. Emirates flight with ‘EK’ flight number operated by
Emirates may be used between Dubai and Europe/Middle East/India and South Asian
Subcontinent where there is no Qantas codeshare service with ‘QF’ flight number
operated by Emirates
Take MH all the way if you can work it. If you don't get what you prefer the first time HUCA.