I couldn’t verify a specific “POP eligibility rules” change for the UK Post Office from the information available here, and the most relevant public records I found point to postal voting rule changes rather than a Post Office grant scheme. The clearest confirmed date in the material I found is that UK postal vote rules changed on 31 October 2023, with the new rules limiting postal votes to a maximum of three years and adding a National Insurance Number requirement for absent-vote applications.

What the date means

The rule change on 31 October 2023 is the point when the new postal voting eligibility framework started. For people already holding long-term postal votes, transitional arrangements applied until 31 January 2026.

Why this may be the wrong scheme

Your wording mentions “POP grant eligibility,” but the sources I found do not show a Post Office “POP” grant or an eligibility change for a grant scheme. The results instead match election-related postal voting rules and older postal-service legislation.

Practical read

If you meant postal voting, the answer is 31 October 2023. If you meant a different Post Office program or grant, the exact scheme name is needed to identify the rule-change date accurately.

TL;DR

The confirmed UK postal-voting eligibility change happened on 31 October 2023. I could not verify a separate Post Office “POP grant” eligibility change from the available public sources.