We should update

https://www.mongodb.com/docs/manual/core/sharding-refine-a-shard-key/

and

https://www.mongodb.com/docs/manual/reference/command/refineCollectionShardKey/#mongodb-dbcommand-dbcmd.refineCollectionShardKey

to more explicitly state that you cannot modify the index type as part of this operation. In some versions this will succeed (SERVER-69220 will block it), but introduce sharding metadata inconsistencies.

For example, users should not refine from {a:1} to {a:"hashed",b:1}. As this would make the routing table not match the physical location of the documents.