An update: we have received medical examination, police certificates and Assistance of support request on 3 October 2019, more than two months after the Acknowledgment Letter from 27 July 2019.
Assistance of support was submitted on 10 October and approved on 5 November. Medical examination and police certificates were already submitted by that date.
2nd installment of the visa application charge was requested on 22 November, paid on 25 November in the morning, visa granted the afternoon the same day.
In conclusion, the conversion from visa 103 to 143 was smooth and the queue date of the original visa application was counted for. Overall good experience with the Department of Human Affairs, except for the delay between the application acknowledgement and the medical examination requested.