The Upgrade MongoDB to 2.6 document does not mention Upgrade User Authorization Data to 2.6 Format is a required step. If a customer is upgrading a sharded cluster with authentication enabled and has users created in MongoDB 2.4, then Upgrade User Authorization Data to 2.6 Format is a must. Without this step, any user administration operation will produce an error like the following:

Error: User and role management commands require auth data to have schema version 3 but found 1