It depends on a lot of things, basically if they look darker now, if you can see a crack between the filling and the tooth(the doctor may see this better)It may hurt also, but not necessarily.....
You can replace them with a resin witch is white, you wont be able to notice the difference with your teeth, better esthetics on those type of materials. they could sting a little for a couple of weeks, but that would be normal also, it'll go away......