If Qantas cancels your reward flight, they will put you on the 'next available', but there has to be an Award seat available. Flights to the USA are heavily booked (I understand), so there may not be an Award seat for a day or so - maybe longer. You could have a look now at Award availabilities for the week after your departure; may be OK but I think they would probably diminish over time. I'm talking Economy here - if its business, you'd be very lucky to get another J award seat anytime soon, unless you were lucky.
If you were to book an alternative yourself, I don't think Qantas would have any obligation, or inclination to re-imburse you. The date of flying is not actually part of your 'contract' with the airline.
Travel insurance is the way to go - essential for any trip I think, but more so in the current environment. Read the PDS with a flight cancellation in mind, and no quick replacement flight offered.
Sorry to not be more positive, but its not a great time to fly on points ATM, I think and you have to be prepared.