I don't work in the mobile industry, but if the iPhone 5/4S/past models? support UMTS/HSDPA/HSUPA at 900MHz and Telstra decides to then put 4G on that spectrum (right word?) - would 4G suddenly work on those devices, with some sort of firmware update, or does it require specialised hardware?
Telstra will roll out 700mHz in future from what i've heard (not 900). LTE isn't just about the frequency, it's also about the bands as well.
LTE is different from UMTS yes (although it's an evolution). If your handset doesn't have the right radio support, it's not going to work (even if it is a re-used frequency from 2G/3G days).
There should never be a deployment of UMTS/HSDPA 3G in the same frequency as LTE anyway.
Keeping in mind, there are 3 different iPhone 5 versions for LTE. One, for AU/AP, Second for Europe and Third for the Americas. Whilst the 2G/3G GSM/UMTS systems will work globally, there won't be LTE worldwide just yet. There's talk of a common roaming frequency in LTE Band 3, 1800mHz, but that's a way off (and rumour has it it's available on the iPhone 5, but has not been 'switched on' as they have not gotten regulatory approval, nor is it used widely yet).
This is the issue with emerging technologies. Competing standards, and having to cope with different spectrum licences globally for carriers.
LTE is going to make it a nightmare for our Customer Service staff trying to help customers with coverage and handset compatibility. I'm glad I passed the CS arena so many years ago