If an existing record contains an earlier code in 040 subfield $e, such as "dcrb" (Descriptive Cataloging of Rare Books) or "bdrb" (Bibliographic Description of Rare Books), and the description is being updated to DCRM(B) standards, delete the earlier code and add the "dcrmb" code to the end of the 040 field in subfield $e.

See also:

Appendix A:  MARC 21 Descriptive Conventions Code