Your upgrade to JFO is based on the requirements at the time.
If you are on the -400 you will upgrade on either the Airbus (330 or 340) or the 777. It would be most unlikely that you would stay on the -400. This is mainly due to the fact that you will do very little sectors.
If you join on the Airbus chances are that you will upgrade on the Airbus.
There is a request form to select your prefered option, but I don't think this carries a lot of weight.