you'll need a new gauge cluster unless you can figure out how to 'change' the one you've got. i heard once that someone in europe had figured out how to do this & was offering to render the service for a fee, but never heard more about who it was. anyway, if you looked at the printed circuit boards inside a set of kph & mph clusters side by side, you'd prolly see a lone, changed resistor value somewhere that 'tells' the firmware [on startup] whether to operate in kph or mph. either that, or the unit recognizes a [one time] moderately complex button press initialization sequence that allows for factory/authorized shop setting or changing between the modes.
since nobody really knows for sure. the easiest way is to just buy a new one here or off ebay. i've seen them around for about $250 USD.