Does an already placed tooth crown need to replaced if the endodontist needs to drill through it to do retreatment?

Depends: If the access hole through the top of the crown is relatively small and the tooth and crown is stable, then the crown does not have to be replaced. The access whole for the root canal will be sealed. However, your dentist will determine if the crown will need to be replaced.

No: Unless there is something wrong with the crowns.

No: The access hole can be sealed adequately after root canal treatment, with a filling.

Yes: For a good long term prognosis, yes. You may have the access closed with a filling in the short term to be sure any symptoms from the root canal infection resolve. The crown should eventually be replaced to ensure a good seal. Otherwise, leakage can occur between the filling and crown.

Usually, not: Usually a root canal can be done without having to replace the crown. The existing crown do not need to be changed if it has good marginal seal.