It seems one can now switch Telstra on contract plans to month-on-month plan via https://www.telstra.com.au/mobile-phones/sim-only-plans
When prompted select Upgrade Existing service.
The plan change fees are automatically waived off. No need to contact customer care via chat.
Note: If one time fees are waived-off then you would see $0 as the one time fees in the final summary page.
If I changed to a current plan from my grandfathered plan I would be paying more for far less data & far less inclusions on roaming. Please check carefully that the deal is better before you move to these latest plans.