"With especially rare or important watches, where every scratch tells a story - like the Rolex & Smiths that summitted Everest or the Omega Speedmasters worn to the moon - no matter how badly the glass is scratched, it should definitely be kept in place to preserve the iconography. The cross-sectional shape of vintage watch glasses was often altered from era to era by their makers; the more important the watch becomes, the more such details matter. But if you have a good, straightforward vintage watch that you want to wear, a badly-damaged glass should be replaced so as to preserve the interior, and this will have no adverse effect on the value."