The problem has nothing to do with the knife height. If you set them up last year and have not done that much chopping they should be fine. If the knives still are in the holders completely then they would be the same as new knives in height. You just would have more knife sticking out the bottom of the holders with new ones.
You are plugging behind the screen. That is caused by the corn being too wet. I bet that if you check some of the corn in the green spots it is still almost in the milk stage. That will ball up and plug the chopper up. The fact it is pulling hard should tell you something. IT IS TOO WET!!! LOL
Also I make earledge every year. I never have made it at 35-40% moisture. Your feed guy is book smart but common sense dumb. Wait until your corn is a little drier. If you do not your going to break something on your chopper.