401(k) Eligibility - 500 Hours

I'm hoping someone can help me here:

• 401(k) plan excludes employees regularly scheduled for under 500 hours per year, with a failsafe that if such a person actually exceeds 500 hours in an eligibility computation period, they become eligible. This was done to avoid LTPT eligibility tracking, i.e., anyone with one year of 500 hours will be eligible to defer, so will not become an LTPT employee.
• Plan also provides that 500 hours in an eligibility computation period is an eligibility year of service. 
• BIS is 500 hours. 
• If someone moves from an eligible to ineligible class, they may no longer defer as of the date of the move. 
• An employee has worked full-time for several years, and will have well over 1,000 hours in 2024. As of 1/1/25, they will move to part-time, scheduled for under 500 hours per year. Assume they actually work under 500 hours in 2025.

Is it permissible to move this person to ineligible status starting in 1/1/25 (the date they are no longer in an eligible class) such that they are no longer eligible to defer, despite having satisfied a year of eligibility service?

If not, what about 1/1/26 after they have a one-year BIS in 2025?

I don't believe you can exclude people with more than 1 year of service unless they are in an excluded class of employee that is not a disguised service condition.

Thanks. That was my initial concern as well, particularly with the 500-hour year of eligibility service definition, although it seems odd to me that an employee would be able to continually defer through consecutive breaks-in-service. Maybe it's not. 

We routinely design our new comparability DC plan documents to require NO hours of service and NO last day of the year employment once an employee has satisfied initial eligibility conditions and has become a participant. With a new comparability plan, it is relatively easy to pick and choose which employees (if any) who terminated during the year will actually receive an employer contribution. If the plan is not top-heavy, the employer can even pick and choose which employees get allocations even among those employees who did NOT terminate. And even if the employer chooses to provide an allocation of the employer contribution (to pass 401(a)(4)), often vesting will mean that there is a relatively small net cost to the employer. This way, we don't worry about BIS, and all employees, once they are participants, can elect 401(k) deferrals for the entire term of their employment.