It's not normally a simple task. There are rare instances when amp modules can be swapped or boards were masked for additional parts. The problem with that can be that the power supply is not capable of delivering the extra current. At any rate, the audible difference between 75W & 100W would be negligible and probably not worth the effort.
Edit: Can you upload model/schematic information? That would give us a better idea of what we are dealing with.