I'm a newbie at PSO2,

I know at level 30 you get to choose another class, let's say I'm a level 30 hunter, I chose to be a fighter(the dual blade thing class), while doing a run, can I still switch weapons like, dual blade then sword or vice versa?

Also, if I choose to be a gunner? Can I switch from dual machinegun then to sword or vice versa? (still while doing a quest or dungeon)

And can anyone tell me, what really can you benefit from the sub-class? Because I've tried, main class = hunter, sub-class = machine gun thing class.. but I can't switch from one weapon of the other class to the other.. T__T