Would like to clear a few misconceptions present in this thread:
The replica objects are still there, you should notice that your used disk space didn’t drop by half. The partition’s objects just aren’t counted during certain state transitions to prevent over counts (by arbitrary decision it was decided to be better to under count records during migrations than to over count).
This KB article is a bit dated - the rec_refs stat no longer exists and was also described incorrectly in article.
Replicas are loaded from disk.
This is really old information, this behavior was change with 3.5.8 release (migrations were much worse before ;)).