While I always enter my address on the Qantas.com booking info, I haven't ever updated the ETSA for US trips. Always took it as valid for two years and I need to get back their web site for a few end of year trips. My ETSA approval date ends mid-year.
I do carry a print out of the ETSA approval when I head to the US. The Check-in desk always "sights" it but never have inspected it.

The last 5 trips have been different from the original booking. No probs going through. They just check my fingerprints against the first entry. And this has been landing in LAX or SFO.

Cheers.