If you use the convention that positive voltages are always above ground and negative below ground, then the conversion from NPN to PNP is exactly like a mirror flipping the circuit upside-down - positive supply becomes negative.
In a circuit where 'ground' is the negative rail then the PNP treats the positive supply in the same way an NPN treats ground. For instance a PNP can switch a load on the 'high side' whereas an NPN switches the ground (low side) of the load.
Hope that less confusing rather than more confusing!! Basically in a PNP all voltages and currents swap sign from the NPN case.
There is a similar relation between n-channel MOSFETs and p-channel MOSFETs BTW