It's a bit of a dodgy game to play because you are effectively tampering with the odometer. I was told by someone local that seemingly serves the majority of the local importers ( ) that they could take the odo for a couple of days and basically pulse it right round to the desired display for miles. Getting it to display the miles light indicator instead of km is the easy part, as documented on the http://www.trlperformance.com site.
With my silver supra (which I imported myself), I chose to convert to read up in miles (and show mph, derestrict; all are functions of a decent chip for conversion) on the odo but just leave it at that and obtained a documented mileage certificate from the chip supplier to show the details of the car and when it was converted and also how to calculate it's current mileage from what was displayed - the difference between what's on the display and what that converts to in miles, so therefore for me it was 45,000 minus 17,000 = 28,000 miles (rough figures). This way you're not tampering with the odo and you have documentation to prove how you calculate the mileage and what's on display. I also had my original jap auction sheet and Jap service history to back this up.
Indeed, jap imports don't leave Japan clocked - unscrupulous UK businesses clock them when they get here.
Hope this helps,
Brian.