I know that a QACA/EACA cannot be added to an exitsing 401(k) during the plan year. My question is can an "unwind" feature be added to an existing QACA/EACA during the year?

Any thoughts are greatly appreciated.

I don't see a problem with adding the unwind withdrawal feature in the middle of a plan year.

Is that really right that one can't create an EACA in the middle of the plan year? I understand that becoming a safe harbor plan must be done as of the first day of the plan year, which would cover your statement about QACAs.