Many (most?) trailer axles are bent slightly from the factory, so they make a frowny face. If you just flip it, it'll be smiling--which is the shape they take when they get bent from overloading.
The "right" way to do it is to buy new spring perches, weld them on top of the axle, then reassemble. The axle itself stays the same orientation it is now, just with the springs on top now.