you should not change your grip....it's been my experience that arching your badk doesn't really help that much, but some swear by it. It's really up to you bro.
don't worry too much about the arch.try to dig your shoulders in the bench and push with your feet, using the arch is supposed to make it less distance you have to push the bar.when you bring the bar down,try to keep your elbows tucked and lower the bar to the spot below yournipples ,like your upper ab,then push the bar in a straight line UP...not up and back over your face.the arch doesn't help me either,but alot of people like it.
I agree with the Mann. Most try to arch big to shorten the push. I personally am not capable of a big arch without lots of back pain and cramping. The bigger guys with big stomachs and shorter arms seem to have the best luck at it, just for getting the big mubers up. They end up almost doing a lockout of sorts
a way to help build your arch is to get a small ball or rolled up towel and place under your lower back while benching. gradually progess to a larger object when your feel comfortable to do so