here's where you download Telstra software
https://www.blackberry.com/Downloads...149E92A551338E
This is the OS, not the firmware, (so you'll still get the rogers startup screen etc).
You'll need the Blackberry desktop software
I believe you will need to delete or rename the VENDOR.XML file located at:
C:\Program Files\Common Files\Research In Motion\AppLoader\VENDOR.XML
First, install BlackBerry OS 4.x but do not run BlackBerry Desktop. Next, delete or rename the VENDOR.XML. Finally, run desktop manager to install.
(I haven't tried this, I flashed my Telstra 8100 with vodafone netherlands OS4.5 a while ago & this step wasn't needed).
I hope that helps.