From Jetstar's deal page:
Get one fee-free date change on new domestic flight bookings for travel until 15 December 2020*. If you have a voucher issued as a result of COVID-19, it can now be used across multiple future bookings.
the asterisk (*) just links to something about Club, but I'll include it anyway:
*Club Jetstar has an ongoing annual membership fee of AUD $55. See the Club Jetstar terms and conditions for more details. The annual membership fee is non-refundable. Price based on payment by POLi, Jetstar voucher, Jetstar Gift Card or Jetstar credit cards. Discounted member fares, bag and seat selection available on eligible flights only (excludes Jetstar Holidays).