I don't think it's rude to ask, if you're prepared to accept a no.
At least in New York, Cantonese restaurants are often willing to do that sort of thing, if it's "can you make the Chicken XYZ without snow peas" or "I'll have the thus-and-such, and would you be willing to add asparagus?" rather than requesting a special sauce or a kind of preparation they don't usually do. I think that's partly because the stir-fry process makes it relatively easy to add or subtract a vegetable from a dish.
Something like "I really like the Special Something that you used to have on the menu, can you still do that?" is easier than asking for a change out of the blue, because there might well be someone there who knows exactly how to make it. I had good results with that at one place for a year or more after the item came off the menu, though it never went back on. I think the trick, as other people have said, is to ask rather than demand, and have a fall-back, whether it's being okay with the unchanged version of the dish, or something else from the menu, or a polite "Thanks anyhow" if you're calling about take-out.