The standard BRB rules for jetbikes are that they can move 12" and fire any weapon, so I think its unlikely that destroyers will change too much, especially since they're meant to be fast moving heavy support for the necron army... which is what they are now. I have no idea what the gauss cannon would become in a new codex but as long as they stay jetbikes it doesnt matter if it becomes RF or heavy instead of assault since they ignore it anyway. If anything I would think that it might change to something like heavy 2 AP3 but they should still fill a similar role whatever happens.