A friend had to have their digital odometer replaced on a car they bought in Canada (Kilometers) to show miles when they brought it back to the states. It was quite a spectacle, a Dodge representative flew down with the new odometer with the mileage preset on the new gauge that was indicated when the request was made. Of course my friend continued to drive it for about a month till Dodge got it all together. Anyway the Michigan rep watched as the dealer installed the new odometer. Dodge didn't trust a dealer to install a new odometer and preset the mileage, guess this says something about Dodge dealers.
I guess this wouldn't be much of an issue on an older out of warranty tractor so a new gauge could be bought and installed by the owner. A note could simply be made in the owners manual stating the original hours on the old gauge and date it was replaced OR if the hours count when the key is on, just hook it to a battery and let it run 24/7 till the right hours are indicated, then install it. Personally I prefer a gauge that count revolutions of the engine and then calculates hours based on 540 PTO speed of the engine. Hours as idle just aren't the same as working hours.