Amending to change definition of comp to plan year comp

[R and R]

My client has a fiscal ending 9/30 but their 401k plan year is calendar. We started a cash balance plan for them and wrote the plan document to use fiscal year comp, thinking that this is what they do for their 401k so it would simplify things to use one census. Also, because the two plans are combined plan tested. 

In February after the second plan year, we discovered that they are actually giving us calendar year comp and that the 401k plan document does base contributions on calendar year. Can I now amend the CB plan retroactively to use calendar year as well? Or is it too late to do that, being after the plan year end?   (The differences in comp would only affect NHCEs since the HCEs all earn over the 401a17 limits.)