Is tissue conditioning the same as a reline for a complete denture?

No. Tissue conditioning (D5850 or D5851) is a temporary lining. The purpose of tissue conditioning is to
prepare unhealthy soft tissues for a final impression for some type of definitive, removable procedure, such as a complete or partial denture. Tissue conditioning provided over limited time aids in the healing of unhealthy ridges before taking final impressions for a definitive removable restoration.