As can be seen by the last few responses, there is no end of "easy" ways to alter the mileage on cars. However, what you cannot alter easily are the MOT records and original service documentation, especially as the MOT certificate information, including I believe mileage, has now all been loaded online by DVLA.
I intend to ask my local service agent (a well known, trusted, specialist in Lotus) to verify the mileage on the Stack cluster (I will produce a letter that he will sign and stamp and I will keep with all the other documentation i have for the car) and the date it was "swapped". If I can, I am happy to "set" the GARW mileage to match the Stack on removal and also get that confirmed and documented. In all, it will take me about 15 minutes to create the paperwork, max.
However, if I ever sell the car, I think the GARW will be a very attractive mod and I will throw in the original Stack unit too. I may be being niaive, but I believe an open and transparent, documented approach like this will allay any fears from future buyers and the MOT records will validate the figures too etc.
I think we're worrying about nothing, but then as i said, I might be being niaive. I bought my VX to drive and enjoy, not as an alternate investment / pension strategy 