There are pros and cons to replacing existing amalgam filling with white fillings. Some of the pros to removing amalgam fillings is to achieve a more natural look. Some cosmetic dentist state that it also reduces the change of undetectable tooth decay.
Some of the cons to this issue. Conservative dentist agree that every time you remove a filling you are also removing a bit more of the tooth. Also each time you remove a filling to replace it with a new filling you are running the risk of killing the nerve of the tooth which can lead to more expensive and complex dental treatment. There for a filling should not be replaced unless it is damaged, broken or there is new tooth decay around the edges or underneath the current filling.