On setFCV=3.8 on the config server, after all the shards are in fcv=3.8, the config server will call listCollections on each database’s primary shard and create an entry in the sharding catalog for each unsharded collection and views. It will also make sure that if the primary shard doesn't own a chunk for a sharded collection, it should have the indexes and matching collection options with another shard that has chunks.