32 CFR §2002.36

1. (a)Agencies must review documents created prior to November 14, 2016 and re-mark any that contain information that qualifies as CUI in accordance with the Order, this part, and the CUI Registry. When agencies do not individually re-mark legacy material that qualifies as CUI, agencies must use an alternate permitted marking method (see § 2002.20(a)(8)).
2. (b)When the CUI SAO deems re-marking legacy documents to be excessively burdensome, the CUI SAO may grant a legacy material marking waiver under § 2002.38(b).
3. (c)When the agency re-uses any information from legacy documents that qualifies as CUI, whether the documents have obsolete control markings or not, the agency must designate the newly-created document (or other re-use) as CUI and mark it accordingly.