You'll have to check the terms and conditions of the voucher - at least one page on the QF site says vouchers are not transferable - they must only be used for passengers on the original booking. Note also you may be required to purchase a fare equal or higher than the fare you originally paid. So if you snagged a $3499 return to London in premium economy, but those fares are now $5999, you'd either have to fly a very expensive economy fare, or pay the fare difference to the new level.
Check the terms of the voucher as some of them seem to vary slightly. The main flight credit terms and conditions are here:
Qantas Flight Credits | Qantas, the only difference with CV-19 is that the change fee will be waived (but not the fare difference).