I took the matter up with Telstra Countrywide on 30 June. They weren't encouraging but they did know what they were talking about. They said that the mini-MUX in my area was full and a new one would be installed in late August. On 1 October, they informed me that this work had not been done but they were optimistic that it would be carried out later in the month. Since August they had streamlined their procedures and it was now actually possible for me to place an order for ADSL while ISDN was still connected, although through CountryWide rather than the normal channels. Moreover, it was now possible to convert from ISDN to ADSL in the one procedure. They actually called me back when they said they would and took an order from me on 5 October, albeit without allocating an order number or anything like that.
Rang them back today, 25 October and spent most of the afternoon on hold (but, thanks to the hands-free kit, still able to work), while they attempted to find my order. Apparently, some special privilege was required to find it and even more to interpret it. They weren't optimistic, saying that the exchange has no spare ADSL ports.
Now they say they can process the conversion and scheduled it for the morning of 8 November. Given the foregoing, you can probably guess why I'm not jumping for joy.
Tested the ADSL modem. Couldn't get it to work.