Coming from japan its most likely going to be 100v so you'd need a step down transformer. 115-100v. unless you can find a European version of course. I'm not sure why your apposed to a step up/step down transformers I've used them in the past for japan 100v gear with no issue as long as you buy a good quality one higher then max power demand.
changing the voltage for the until should be easy if there is a switch to allow that i.e. transformer taps for the different voltages. If there is not a switch then there still could be a small possibility that the installed transformer has the taps internally (but not connected) that may need to be connected. if not then the whole transformer would been to be changed. Either way if there is no switch to change voltages then its probably a technician level change at the minimum. lets not even get into the warranty issues doing this outside of the factory if new. IMO if there is not a switch then its probably best to use and external transformer.