It is indeed safe to replace amalgam fillings. Some patients are concerned about replacing amalgam fillings. In particular, they're often concerned in cases where the metal has expanded significantly, leaving a portion of a decayed tooth exposed. In these cases, your dentists can effectively remove the remainder of the filling, in order to place a white filling in the space. Assuming they can salvage the tooth, the dentist will be able to place a new filling in the hole left behind.