Replace a Filling prior to a Crown?

My dentist wants me to replace an old silver filling with a white one (not sure of the technical term) because the old filling has caused some cracking/fissuring. He also says I should get a crown on the tooth in the near future. If I get a crown, is it necessary to replace the old filling at all? I thought the crown would do the job of a filling too.  I asked him and he said that yes, he had to replace the filling prior to the crown.

I've had 2 crowns done before and they've grinded the tooth down to very little and it seems like a filling would just disappear at that point.

[Jerome Tsang, DDS]

If a crown is needed, he may be referring to a core buildup as a filling.  It is essentially the same procedure but we normally remove old fillings and replace it with a core build up (filling) to replace any damaged parts of the filling plus to inspect and make sure that any crack or fissuring of the tooth doesn't extend beyond the original filling.  Sometimes I may leave an old filling in if its still relatively intact or if it looks very deep and removing it may cause nerve damage and result in a root canal.

[Dental2010]

Hi,
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