John,
The A320/19 is due to be the major shorthaul A/C. The 757/767 fleet is on a rundown but in my experience of fleet rundowns it could take about 10 times longer than advertised!
The 777 is already taking some of the 400 routes. However the 777 will never replace the 747 on certain routes.
Hope this is of help.
Regards
Exeng