Why Replace a Filling?
Fillings don't last forever. They can become discolored. For example, amalgam or silver fillings corrode and tarnish. Composite, tooth-colored fillings pick up stains, and yellow or darken over time. When you chew, your teeth and any fillings in them are subjected to tremendous pressures. Even if no other problems develop, some fillings will wear out over time and will need to be replaced. A filling will need to be replaced earlier if it falls out, leaks or cracks.
Food debris and bacteria can seep down under a filling that is cracked or leaking. Since you can't clean there, the bacteria feed on the food debris and form the acid that causes tooth decay. Decay under a filling can become extensive before you notice it or it causes you pain. This is why you should have your fillings checked regularly and get them replaced when problems are found.