The codes used in SNOMED International are present in SNOMED CT in the 900000000000498005 | SNOMED RT ID simple map reference set | which links the old alphanumeric codes to the SNOMED CT concept identifiers. For further information see the specification of Simple map reference sets.
The 900000000000498005 | SNOMED RT ID simple map reference set | can be used either to allow recognition of legacy data by SNOMED CT retrieval tools or to enable mapping of the codes and storage of the appropriate Concept Identifier.
The meaning of coded clinical data encoded using SNOMED International is maintained using SNOMED CT mechanisms that support concept permanence and version control. Even when a concept is retired from active use its code is never reassigned to another concept.
Concept permanence ensures that codes assigned in SNOMED International are retained, accessible, and not reused.