changing eligibility - when does a participant enter the plan?

We have a calendar year 401k plan that through 12/31/23 had a 1 YOS/1000 hours requirement to enter the plan, with immediate entry upon satisfaction.

Effective 1/1/2024, the eligibility was changed to 1 YOS/500 hours with monthly entry.

This is small plan and through 2023, several employees hired in 2021 and 2022 did not meet the 1 YOS/1000 hours in any year (or calculation period).  But they did have over 500 hours in those years.

Ignoring the LTPT aspect for now, would they be "regular" participants as of 1/1/2024 (since they previously met the new 1 YOS/500 hours requirement) or would they still have to meet the new requirement (1 YOS/500 hours) in 2024, and become eligible 1/1/2025?

I think it would be 1/1/2024 entry since they met the new requirements on 1/1/2024.  

Thank you for any replies

I think it depends on the plan/amendment language. You need to examine the year of eligibility service definition and the eligibility computation period for such. If the language doesn't (explicitly) support using 500 hours in a computation period prior to 2024, I think these people come in 1/1/2025.

Also, isn't it a little late to be asking if these people are 401(k) eligible 1/1/2024? If language is sufficiently vague and reasonably interpreted by the Plan Administrator that 1/1/2025 is the entry date, then that is their right.