What happens to the 763's will depend entirely on QF
's domestic business plan. They'll buy the replacement which suits it the best.
In my view it seems Qantas has segmented it's market over the last few years into two basic camps (It's much more complex in truth, but this is the watered down version). Basically they've had:
1."Red E-Fare" passengers to counter Virgin.
2."Traditional" passengers (eg. Business Travel and Agents)
It seems Qantas is moving towards a strategy where as Qantas Mainline will offer a premium service (frequency, flexibility, meals, lounges etc) at a premium price aimed at the second group above.
In turn it's establishing Jetstar to handle the first group.
That means fewer passengers on Cityflier services - hence you would need fewer seats per flight. In which case QF
could probably get away with buying more 738's to replace the 763's. That would leave them with a single type domestic mainline fleet (ie. 737), which economically seems the way to go.
In terms of business strategy, if you think of Toyota it's the same strategy they use very well. Toyota offer their Lexus brand for the wealthier premium car market, while their Toyota brand offers cheap, reliable motoring for the masses. That's where QF
Domestic is heading.