After he does the root canal does dentist replace crown?

Depends: Did the dentist made a hole through the top of your crown to do the root canal? If so then, and the rest of the crown is ok and there is no decay anywhere else on the tooth, a filling can be placed in the hole that was made to do the root canal. After the years pass by, this crown may need to be replace or the fillng replaced depending on how it holds up.

Depends: If the root canal is done through the top of the exsisting crown and the crown is still stable, you may not need to replace the crown. However if the crown is bad then a new crown will be needed.