Ok, some basics. Allegro is netlist driven. That means you can't force a change without a representative change to the netlist.
The *right* way to do it is from the schematic tool. You always have the option for any part coming in to have alternate padstacks and then you simply select the one you want.
But let's say you want to break the system and do it anyway. :) If you have the proper license, then simply click on Logic->Part Logic and edit in that pop-up box. If you need to change net connections you use Logic->Net Logic to alter the pin connectivity.