Thank you all for the answers!
Jim, my question was if the odo is showing the ebay cluster mileage or the car's. I understand that it is the cluster's one.
Now, to tell you my story:
-I bought my 62 k miles Jag back in 2010 from UK(miles cluster)
-in 2013 I changed all the driving post from right to left, and now my car is a LHD, working perfect since that.
-I want to change also the cluster that now is in miles, with one in km. I found one on ebay, being the part number matching my chassis number (no VCAT on my car).
My questions, if somebody had previous experience are:
-if I will just swap the clusters, the ebay one, will show it's old mileage?
-it will work, it's just plug and play?
-if I will swap the motherboards, as ericjansen just did, I will have a km face, but counting in miles as my(existing car's) motherboard is programmed?
Any other ideea?

It is not a problem to reset the mileage,the law is more permissive here, as I have the proof that my mileage is lower than the one on the ebay cluster (mine is now 85kmiles=136 km, the ebay one is 196 km, so I must to change it). On my Land Rover, if the cluster and BCU are not syncronsed, the screen will flash, same here?