The modifications that a watch most frequently undergoes after purchase are those at the request of the owner. Modifications have always appealed to the human mind over the centuries: the Romans modified chariots according to their use, gladiators modified weapons to cope with tough fights against ferocious animals or ruthless opponents, and then cars, motorbikes, planes, speedboats, etc. have always been modified. The art of customisation has always reigned supreme, so why keep watches out of this craze? In fact, how many times have we come across models that were born 'normal' and then handcrafted by complacent watchmakers, goldsmiths, engravers, quadrant-makers, at the precise behest of their owners? Many times I must say, and often just a few days after purchase, perhaps with soft interventions, first of all the strap, to continue over time with the dial, the element mainly attached, or the hands (the red seconds spheres are very popular), to continue with much more difficult things such as insertions of precious stones on the bezels, or engravings and skeletonization of the movements. But the most singular thing is that many of these 'modified' watches passed from hand to hand and ended up in the hands of new owners convinced that they had come into possession of rare and unique pieces, made by the House for this or that anniversary or, even, for this or that country or even for various Armed Forces: nothing could be more false in the vast majority of cases, nothing to do with the originals, truly limited edition pieces created in honour of events or famous people. "Mine is a special piece, a rare edition of a normally produced reference or model, bought by a collector or at an antique market or in a remote little shop uncovered during an adventurous holiday!" These stories often occur, and so disappointing the unfortunate person with a ... "Look, yours is a very normal watch modified at the request of an eclectic previous owner" must be done with style and courtesy, to avoid the unfortunate person having an attack of bile with dangerous consequences. Do you remember the period when considerable quantities of watches from a well-known Swiss manufacturer, equipped with dials in frivolous pastel colours, invaded the wrists of many VIPs, many of whom were convinced that they were factory originals? Just a phenomenon linked to a moment that was very trendy and rewarded those geniuses who made money out of it by disappointing those who took it all in one breath. So, as usual for good advice, don't make any changes to your watches if you really care about them, as any even small change will downgrade your good to a non-original product. If, on the other hand, you buy a second-hand watch, make sure by asking a dealer, or by documenting yourself through the dozens of publications, whether it really is a special, numbered or limited piece, check that it has the regular documentation, check the reference numbers engraved on the back or on the case middle, through which you can be certain that it is the model you have before your eyes. It is clear that with modifications on watches you will not risk penalties like those for cars or motorbikes, but it is also true that no one will recognise as original a product that has been 'retouched' and perhaps damaged by improper treatment.
