I don't particularly like to customize vintage watches, but the customer is always right, and anyone has the right to do whatever he wants with his personal property. In any event most of the time such customization is simply making a fanciful dial (adding diamonds, a name, changing to a non-original color etc.) and that can be undone in the future if a subsequent owner doesn't like it. I do NOT do things like converting vintage movements to modern quartz, or anything that might be misrepresented as a rare original (e.g., fake "Pacermatic" watches).
In years past I did do some customization of modern Hamiltons. When they were still selling a yellow-plated Venturas as part of the regular line I had some silver dials redone in black with the original markings ("Electric" and "Pat. Pending."), and painted the second hand tip red. On the wrist these looked totally original but they could never be passed as originals since they were plainly gold-plated and not solid gold.
Another special customization I did was to take a few Flight II replicas and convert them to Flight I replicas. This entailed replacing all the dial markers as well as refinishing it with a different pattern. I also capped the seconds pivot since originals did not have second hands. These came out looking very cool. I occasionally wear mine to watch shows and have often been offered huge money by people who see it on my wrist and figure it must be original since Hamilton never reproduced the Flight I. Then I smile and hand it over saying "sold." It's fun to see their faces when they turn it over and see a stainless steel rather than 14K gold back..... Then again, just a couple of weeks ago someone sold one of my "Flight I" replicas on eBay (fully and accurately described) and it sold for $1,000!!!
(Just for comparison, here's the regular version prior to the transformation of the dial.)