Benefit accruals after lump sum?

Can a DB plan be amended to permit in-service lump sum distributions and also cease future accruals for any person who elects to take a lump sum?

If the late retirement provision use the greater of the actuarial increase or the age/service benefit, the actuarial increase will almost always be higher.  Therefore, if the participant receives a lump sum, and the plan offsets future accruals by the actuarial value of prior distributions, there will almost always be no future accruals.  IOW, the value of the prior distribution will exceed the value of the additional age/service accruals and no new benefits will be payable.  

This would involve active participants who have not yet reached NRA. 

My understanding is they would like to offer active participants the opportunity to receive a lump sum, and if they do, they will no longer accrue benefits. For those who don't take the lump sum, they would still accrue benefits. The plan is partially frozen, so only the compensation element of the benefit formula can continue to grow. 

Have I missed something?  What is the distributable event?

As Dave alluded to, you can't just offer lump sums to actives because "they would like to".  There has to be a distributable event - attainment of normal retirement age, plan termination, death, disability, retirement, etc.  I think you can slide down the in-service distribution age to 59.5, but not younger than that without terminating the plan.    

Regarding freezing accruals for those who take the lump sum prior to NRD, it seems like as long as the effect of the amendment is non-discriminatory, you could do it.  I would make sure you accurately disclose everything to the participants, then it is their choice, but it feels pretty scummy.  You would also be required to show the plan is non-discriminatory every year thereafter since they froze accruals for non-excludable participants.