Minimum Entry Qualification into the DP Revalidation Requirements shall be as follows:

The minimum entry requirement is a DPO Certificate issued by the NI. The original DPO Certificate should be presented at the DP Centre where the Revalidation Course is completed. The Revalidation Course may be completed up to six months before the revalidation date set by the NI. The course will be valid for a period of 12 months only, in which the DPO must send in an application into the NI. If the participant is completing the course for the first time, there is no minimum DP sea time requirement. If the participant is completing the course for a second or subsequent time, a minimum of 28 DP sea time days will be required to revalidate. This DP time may be completed prior to or after the Revalidation Course. Where a participant subsequently revalidates after taking the course by completion of the full sea time requirement of 150 DP sea time days that participant can then take the Revalidation Course again without a DP sea time requirement.
Legacy and Grandfathered certificate holders will be able to take the course to revalidate until 31st December 2019. After this date, previous Legacy and Grandfathered certificate holders will need to complete DP sea time days towards revalidation.

Source: The Nautical Institute Certification and Accreditation Standard - Vol 1 - Training and Certification - V1 - January 2022