Because downgrades are more difficult after you upgrade the user authorization model, once you upgrade the MongoDB binaries to version 2.6, allow your MongoDB deployment to run a day or two without upgrading the user authorization model.
This allows 2.6 some time to "burn in" and decreases the likelihood of downgrades occurring after the user privilege model upgrade. The user authentication and access control will continue to work as it did in 2.4, but it will be impossible to create or modify users or to use user-defined roles until you run the authorization upgrade.