Yea you can go up in temp to step up, but can't go down. Say do a 15-20 min mash at 145 and then go up to 157 for the rest of the hour. The goal is not to get all converted at 145 though, otherwise like is said above very watery beer.
Reason you can't go down is because two enzymes are at work that have different heat ranges, and because they are just enzymes, they have no protection from the heat. Once you go past their tolerance they turn into spaghetti (called denaturing) and are useless.
Palmer's website how to brew explains as well. Most people pick one temp where both work, between 150-156ish, and then each one competes with the other and gives you the desired effect. I'm pretty sure it makes the conversion slower though, because both are not operating in each's sweet temp spot.