to add to that, if you read the book in the rotom transform area in silph co it explains clearly that rotom will 'crawl out' of the engine of the device and revert to its original form, this is most probably because pokemon diamond and pearl do not have the data of rotoms different forms, (it also does not have girantinas original form, but that's another matter) that is simply made to prevent game glitches and corrupt pokemon and game save files so its good that its there.