If you have old silver fillings, they sometimes look black when you open your mouth or laugh. Some people change their old fillings for health reasons, but it is not proven that they can do you any harm. It is not hard to change old fillings for new resin (white) fillings, but disturbing old fillings (especially deep ones) can sometimes make the teeth slightly sensitive for a while.
White filings not only look much better, they also are less prone to leakage as the resin is bonded to the teeth. If you have a large number of silver fillings it may be more cost effective to do a few at a time.