Here is a thread that discusses some of the differences between the two scanners. Read down a little bit it is kind of buried in the talk about the BC246T.
http://www.radioreference.com/forums/uniden-scanners/36531-bc246t-will-replaced-anytime-soon.html
Here is a website that goes a little more in depth:
Differences between Australian and US Scanners
Here are the frequency coverages for both models:
* (North American Model BCD396T)
* 25.0000 - 512.0000 MHz
* 764.0000 - 775.9875 MHz
* 794.0000 - 823.9875 MHz
* 849.0125 - 868.9875 MHz
* 894.0125 - 956.0000 MHz
* 1240.0000 - 1300.0000 MHz
* (Australia/New Zealand Model UBCD396T)
* 25.0000 - 512.0000 MHz
* 764.0000 - 775.9875 MHz
* 794.0000 - 956.0000 MHz
* 1240.0000 - 1300.0000 MHz
I think the main thing is that the channel structure is different in Australia then it is in the United States. As far as I know there is no firmware update that will make the American scanners work in Australia. If it were me I would sell the scanners you have and buy the "U" models.
One more thing. Here is a link to the website for Uniden Australia w/some more information on the UBCD396T scanner that may be helpful.
http://www.uniden.com.au/AUSTRALIA/p_ubcd396t_index.asp
Hope this helps!
Joey