The problem with running the mower motors at higher voltages is that the field magnetism is set by the permanent magnets, thus if the armature has a higher voltage you will wind up with a "field weakening" situation. The motor will spin faster and yes the magnets will take a beating and bad things will happen.

On the wound motors you will have the field voltage and the armature voltage go up together so yes you will have more power and it will still spin at the same "speed". 48 volts probably would not be an issue, but since you are pulling more power the motor will try to dissipate more heat under load.

This by the way is why I can run the lift motors on 36 volts: They have a series wound field along with the armature, so by increasing the voltage the motor's field and armature stay in equilibrium. One will wind up with a LOT more power and also the motor will dissipate a LOT more heat as it loads up which can melt wires. However for the same amount of power you're using only 1/3 the amount of amps which is why I put a 20a fuse in to protect the motor instead of the 30a fuse. Voltage drop in the wiring is less, and the tractor is more happy.

Until you leave the switch pressed when the deck hits the stops at which point it will rip the mounting bracket off the motor. Unless you reduced the fuse current. But that's life in the fast lane.