How common are last minute aircraft changes on SYD-MEL?
 

Flying to MEL on Tuesday on QF439, listed as a 737-400

An ACARS history search on 16right.com shows a huge variation of 737s and 767s on this particular flight

Although I am of course aware that the scheduled aircraft for SYD-MEL flights would change regularly, how often are last minute changes to the scheduled aircraft type made? eg, having your flight listed a 734, and finding a 767RR at the gate.

I usually don't bother checking what aircraft I'm scheduled on when flying domestic, as I've been on all 737 and 767 variants with Qantas. But out of curiosity, how "likely" is an aircraft change for my flight?

A few weeks ago I was on QF453 and at the last minute it was changed for an A333 to an B763ER.