Well, actually it should be a step-UP transformer, but these are generally inefficient and ruin the sound. The Perreaux people in NZ say, that such a conversion could be done, but only through qualified personel. This often entails the un- and the resoldering of leads on the transformer, but need not necessarily be the case. Perhaps you could unplug the amp, wait a while until the big caps have discharged, open the case and look inside if there is a switch of some sort for this or better still, take the amp to a good tech. At any rate, according to the literature of the manufacturer, this conversion, as you plan it, is possible.
Cheers,
Detlof
Cheers,
Detlof