Updates/findAndModifies that change a document's shard key value require the user ran the operation in a retryable write or transaction so there's either an active transaction or a transaction number to use for a spawned internal transaction that atomically deletes and inserts the document. We can now use the transaction API to remove this restriction, since writes without a transaction number will create an internal session for the transaction so there's no need to reuse a client provided transaction number.