If they are rated the same watts, then just remove the ignitor from the HPS circuit, and voila! its an MH ballast now..

The HPS is under High pressure (duh! ) and so it needs the ignitor to step up the current. The metal halide does not use the ignitor, but the other hardware is the same

All the switchable ballasts do, is have a toggle switch that isolates the ignitor from the circuit.