Mike's advice is usually very good. No reason to think otherwise in this case. And when he says "ERISA counsel", he means exactly that.
The facts you present may indicate a possible error by the plan administrator (perhaps unintentionally). If so, multiple plan participants may have been "shortchanged". Mention that to legal counsel.
BTW, some of the readers here will be curious about the outcome. It's OK to post the results, omitting any confidential details.