Head curves are primarily determined by the operational RPM, impeller, diffuser and volute. If a pump motor is properly loaded or even slightly underloaded it will run at 3450 RPM on high speed and 1725 RPM on low speed. If the motor is overload (i.e. motor too small), slip can increase and thus RPM decrease which in turn can change the head curve.
The wet end and motor are actually two separate machines which are connected by the motor shaft. The loading on the motor is determined by the wet end load and the wet end load is dependent on the speed of the impeller. So the motor is sized to the wet end load and if you have the same wet end, then when replacing the motor you want to make sure the new motor is rated to at least the same THP as the old motor. That way you are guarenteed the new motor will not be overloaded. It is ok to underload a motor but a motor cannot be overloaded without serious consequences.