Switching that often makes it kind of tough. You might consider installing the hardware for them and then only actually use the cover plates when you have to. Most of the time you can probably get by without them, but sometimes you just need that extra measure of threshing if you have green pods. I'm not sure how else to get it without causing splits. Between the proverbial rock and hard spot I guess. As for the lower sieve, we have the adjustable airfoil from loewen. It forms a pretty round hole that can be adjusted just slightly larger than your largest beans. Of course, some unthreshed pods still get through. I would think that this sieve would open up plenty for corn when you make the switch.