The Hadoop in Real World team show how to migrate specific documents when building a new index:
As shown in the other post, we still use a reindex by specifying the source and destination but this time we also specify a query in source along with the term which indicates that the documents with state = ‘ny’ will be filtered from the source.
So only documents with state ny will be copied to the new index account_v3 with this reindex operation.
Click through for an example of how this works.