Is there anyway to switch between presets in Preset mode by simply pushing down on one of the 3 bottom pedals, or will it only activate the new preset when you pull off?
No, it is not possible to change the way the switching works directly on the Nova System. However, it is possible to connect a Switch-3 (http://www.tcelectronic.com/switch-3/) or a MIDI controller, such as the Tech21 MIDI Mouse (http://tech21nyc.com/products/effects/midimouse.html) to the Nova System and use that to switch between presets.
A great way of getting more flexibility from the Nova System is to use an external switch, such as the ones mentioned above, and set the Nova System to Pedal Mode so that you get the benefit of being able to switch the various FX on/off while still being able to change presets on the fly. Please be aware that if you use a Switch-3 you will not be able to connect an expression pedal to the Nova System at the same time.