32 CFR 2400.24: Downgrading.
(a) When it will serve a useful purpose, original classification authorities may, at the time of original classification, specify that downgrading of the assigned classification will occur on a specified date or upon the occurrence of a stated event.
(b) Classified information marked for automatic downgrading is downgraded accordingly without notification to holders.
(c) Classified information not marked for automatic downgrading may be assigned a lower classification designation by the originator or by an official authorized to declassify the same information. Prompt notice of such downgrading shall be provided to known holders of the information.