In these days of belt-tightening, I'm not sure if they would still do it, but I've experienced WDW waitstaff noticing that I wasn't really enjoying all or even just some portion of my meal and offering to replace it with something I would like.
I remember one specific example, at Le Cellier when I ordered their Signature Dish--the Mushroom Filet, with mushroom risotto. Now, I love mushrooms, and I like rice, but I did not like that risotto, so I just left it and concentrated on the meat and veg. The waiter noticed that I wasn't eating the risotto and offered to bring me some mashed potatoes or something. I turned him down because I was pretty full already, but since then I've never been afraid to try something new and always encourage my picky eater husband to request substitutions, if it gets him to try a new dish (don't like sweet potatoes with the duck in France, ask for mashed--at least he gets to try the duck, and he liked it!!)