Changing In-service Withdrawal Conditions--Cutback?

Plan currently allows for in-service withdrawals at ag 59.5 for all sources.

They would now like to limit it to only accounts that are 100% vested.

Is there a cutback issue?

Prospectively you could change it prospectively but amounts available now under the old rule would have to be grandfathered as eligible for in-service. I bit of pain determining the amount eligible until everyone at the time of the amendment reaches full vesting. At least that's my understanding.