The addShard command has two use cases:

• (1) adding new shards to an existing sharded cluster
• (2) promoting a replica set to a sharded cluster

In both cases, it upgrades (or downgrades) the FCV of the new shard to match that of the config server.

When (1) adding new shards to an existing sharded cluster, upgrading the FCV will be fast and succeed because the new shard must be empty.

However, when (2) promoting a replica set to a sharded cluster, the replica set to be promoted may not be empty, so the FCV upgrade may be expensive and/or fail, and may be unexpected to the user.

We should consider not implicitly upgrading the FCV on the replica set to sharded cluster promotion case (instead forcing the user to explicitly match the FCV using setFCV) or adding a mention on the documentation about this behavior.