It's the other way around, Sportsman. To compensate for shorter cranks you should adjust the front triple gear accordingly, and gear them down a bit.
Shorter cranks are much easier on the knees, provided you gear your triple ring down accordingly. Or you can leave it alone if you don't like to spin a bit faster but you'll lose pwer on the hills without gearing down much more than the usual. In any case it is much easier on the knees. My knee pain on my left knee went away immediately after I went fro 175's all the way down to 153's! Although I wouldn't recommend taking a big jump like that for upright bikes. Worked extremely well on my bent though. I'd never go back to loger crank arms. Even the few times I ride an upright with stock 172 or 175's I can feel my knee grating after a mile or so.
I think someone makes adjustable cranks. I've seen them in a catalogue somewhere, I think maybe Hostel Shoppe (a bent bike shop) But putting on shorter cranks is only half the changes you have to make to the overall bike. You'd have to gear down on hills a lot as you lose torque (remember physics 101) going to the shorter ones, but because your knee isn't being wrenched so far with every pedal stroke it is much easier on them. Your cadence will increase a bit but your knees will really thank you!
I got my shorter cranks from a guy named Mark Stonich at Bikesmith.com. He has a very informative web site and loves to talk on the phone.