The most common reason for replacement of all types of restorations in permanent
and primary teeth is recurrent caries. Recurrent caries, also know as secondary decay would be highly suspect in a 20 year old amalgam filling. Prior to placing a crown, it is critically important to insure that no bacteria reside in the underlying tooth structure. The only definitive way to verify this is to remove the old filling, disinfect the tooth and replace it with a new restoration.

Mark Buzzatto