Amending plan to change definition of Retirement Age - Impact?

Plan currently has a retirement age of 55.   They allow installments if you are retirement age when you separate, otherwise lump sum.  Sponsor wants to change the retirement age to 60.  My question is, can that be done retroactively for all current participants?  or must it only be allowed for new monies deferred after the amended change?   It's not accelerating a payment and trying to track what and when someone is paid if they separate is going to be very cumbersome.  I appreciate any thoughts on this. 

You haven't said what kind of plan this is, this is not legal advice but a change in NRA that is more restrictive can result in a cutback. You need to understand all aspects of the plan to determine whether there would be a cutback.  A couple examples come to mind:  Where a plan permits in-service distributions at NRA, changing would be an impermissible cutback of a retirement option, with respect to the accrued benefits and also NRA affects vesting if there is vesting, people fully vest at retirement.    

JA, based on the forum you posted in, is this a plan of unfunded deferred compensation for selected executives?

I didn't notice this was 409A forum!  

This is an unfunded nonqualified deferred compensation plan.  each year the participant can elect to receive their distribution on a fixed date or retirement.  if they separate from service prior to either event, they are paid as a lump sum.   There is a vesting override for age of retirement.

I think you can only change NRA for compensation deferred after the amendment.