If the crown is well fitted to the teeth with no secondary caries suspected below the crown then there would be no reason to replace the crown. Following root canal the tooth would be assessed for its response to treatment for a period of 6months to one year. During this period a semipermanent restoration may be placed in drilled crown space.
If after a year you fell good with the teeth, then a new crown may be an option if you have issues of roughness at the margin of the crown and restoration. An important part is however to make sure that the restoration placed within the crown does not fracture or leak, as this could cause bugs entering the root canal space and causing a few issues in future.
If there is no decay inside the crown, filling the cavity is acceptable.
no they dont replace the crown just fill it.