http://docs.mongodb.org/master/release-notes/2.6-upgrade

1. Some "2.5.x" are not updated to 2.6
   For example:

   Upgrade/Downgrade Path:
   Once upgraded to MongoDB 2.5.x, you cannot downgrade to a version earlier than MongoDB 2.4 via any downgrade path; i.e. you can only downgrade to MongoDB 2.4. If you have text indexes, you can only downgrade to MongoDB 2.4.9 or higher.
   

2. It would be nice if we can also clarify the roles required for this adminitrator user.

   Ensure that at least one user exists in the admin database before upgrading to MongoDB 2.6.
   

• 2.2 legacy user should have read/write permission in the admin database.
• 2.4 user should have userAdminAnyDatabase roles in the admin database.