Switchable by changing wires. But energy saving doesn't exist (nearly ) in my concept. I mostly use then the 125w HQL ; someone in L-G.net has the motto: As brighter as better! (or similiar) He's completely right!
But any "multitap" feature (regardless of mains voltage or lamp wattage) has it's cost: Higher losses or larger size/weight of the multitap compare to single rating ballast...
100/150 watt HPS multitap wattage ballasts.....are pretty tiny LOL, however the MH multitap VOLTAGE (120 volts, 208 volts 240 volts and 277 volts) are pretty big! I actually have a 1500 watt Mulivoltage MH ballast...it's a MONSTER!
By the way, In Israel, Eltam Ein-Hashofet produce a multilamp reactor ballast for 250W lamps that can be switchable from 250W HPS lamp to 250W MH lamp and vice versa by changing wires.