Controlling a DC Motor 48v with a 63v power supply
You actually have a few issues when driving a DC motor with more than the boilerplate voltage rating.


Max Current: At no time should you exceed the maximum current rating of the motor. It will overheat and can in fact demagnetize itself. As such you need a current regulation or limiting circuit to keep the applied current sufficiently under the max.
Over-arcing: When you apply a greater voltage the arcing at the commutator  will be hotter. This will increase the wear on the brushes and commutator and dust production. Motor life will be significantly reduced.
Loss of control issues: You need to take extraordinary measures to account for various additional failure modes. For example, what happens if the PWM driver gets stuck on the ON position, or if you lose your logic supply or the micro/software goes belly up.
Electrical Safety: The difference between 48V and 63V does not sound like much, but it can mean the difference between a tingle and death. If you plan on using larger voltages, extra care needs to be taken to insulate it and, if it is user serviceable, I'd advise extra labelling on the motor to indicate a higher voltage is present.
Mechanical Safety: Driving with a higher voltage will mean higher available torque and speed, especially in a failure condition. You should ensure that such changes are within the limits of whatever the motor is driving and can never cause bodily harm. (Over-torque on a hand-drill comes to mind...)
EMC: Now you are switching higher voltages, you are also adding more electrical noise, not just in the switching, but also in that darn commutator. More significant snubbing circuitry at the motor is likely required.