My understanding is that for the Longines "Tre Tacche" watches (and as is the case for so many watches in the vintage world generally), it was not uncommon at one point for dials to be swapped and put into other cases - for example, because a watch with an amazing dial may not have had the best case, etc. The question is, I suppose, whether that devalues the watch or whether a piece like this can still be valued at his asking price of 12K euros given the very rare dial and great condition of the case.
I would say that now is the point in time in which tre tacche dial swapping is common. I do not know if things have changed in recent years or if it has always been this way. What has certainly not been a constant, though, is the price of tre tacches, especially 35 mm ones. Prices have increased substantially in the past decade. There seems to be a direct correlation between the price of 35 mm tre tacches, and evidence of swapped dials coming to light. I grant that this could be a result of new information provided by Longines' archive, and not indicative of a change. Either way, profit seems to be an undeniable motive.
In the best case scenario, a seller would disclose a swapped dial, and use a dial that is appropriate for the case/age/market of the watch. Unfortunately, as more dials get swapped without disclosure, it is becoming more difficult to determine what an appropriate dial would be. Catalogs offer some clues, but they are not exhaustive.
To me, the value of a tre tacche with a swapped dial is not the same as if the dial was apparently original to the watch. I do not have a rational justification for this opinion, I simply prefer original examples. I like the idea of having a watch that was conceived in the period in which most of its parts were produced.
At first, I thought that appraising a watch with a swapped dial could be done by summing the value of the dial on its own, with the value of the rest of the watch sans dial. Now, I wonder if this is circular since the value of the dial on its own is influenced by the value of complete, original examples.