Techniques are described for migrating namespace content from a source system to a target system. Migrating the namespace content involves copying data associated with a namespace from the source system to the target system in a manner that allows guests on both the source system and the target system to access the data during the namespace migration. Further the techniques take advantage of any replica of the namespace content that may already exist in target system, so that only blocks that are not already in the target system are copied as part of the migration operation. Both the source system and the target system use Content Addressable Storage, and have fingerprint indexes that allow blocks to be located based on their fingerprints. Thus, the target system to which a namespace is migrated may determine whether a desired block from the namespace can be obtained locally based on whether its fingerprint index has an entry for the fingerprint of the block.