These saxophones aren't plated, they are lacquered. Plating is a very thin coating of metal, e.g. silver or gold over the body, whereas lacquer is a clear coat of transparent lacquer, which is a type of varnish I suppose.
Sooner or later this usually wears off. In the old days it was very common, todays's lacquers are harder wearing but are still likely to wear. What happens then is the exposed metal will tarnish . This is not actually bad with brass or bronze, it won't disintegrate like iron going rusty and in fact some people like the dull "patina" that develops.
Relacquering is possible and it used to be quite common, not so much now. To relacquer a saxophone properly it's necessary to strip it right down, remove all the old lacquer, have it relacquered and then reassembled. It's an expensive process for a saxophone that does't actually need a full overhaul.
I have heard of people "spot lacquering", ie polishing the dodgy bits and spraying on a local coat of lacquer, but this unlikely to be an invisible or very permanent solution.
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