I had this situation with mine when got it. can't help with a 'how to do it', as it was pushing nine years ago. But I can say that it only took about half an hour. As to the mileage problem, I've just kept the old 'foreign Devils' speedo. Another way might be to take a photo of your old speedo reading, along with one of your 'new' one (if it aint 0 ),then stick it in an envelope and post it to yourself, but don't open it until you come to sell. That way the date stamp on the letter will back up your claim for the date you changed them.
And yes, I know, you could take photos of any speedo, you've just got to be honest with yourself about this...
Dave.