you do void your mplan ... but many after market tuners have arrangements with certain technicians at specific dealerships that will take care of your car ...
JSN does Schnitzer, i heard through the grape vine that they were doing ECU upgrades to the 335i's and 135i's ... why not a 320d as well ... speak to them and find out ... that way you keep your mplan. But they might charge you an arm and a leg to do this ...