It is true that amalgam contains mercury, but the amount is so low that it does not pose any health risk to patients.  There is a lot of information out there on the Internet; however, I would only trust information from respectable sites (.gov or .edu).  Here is a good scientific article about amalgam for your reading pleasure (http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3388771/).  Even though we do not use amalgam at our practice, amalgam is a good material with a long dental history.  The main disadvantage of amalgam is esthetic.  If there is no recurrence caries and the amalgam is not fractured, I would recommend that you leave it alone.  If esthetic is of great important to you, then replacing amalgam fillings could be justified.  However, if your main reason is that amalgam is toxic, I would recommend that you reconsider.  Ultimately, the choice of replacing amalgam is yours to make.