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It's not too late, time-wise, but you would have to show a substantial change in circumstances of either you or your ex that could not have been contemplated at the time of the divorce. That might be where you would run into trouble. You could easily have figured out that your boys would grow up and go out on their own.
Your retirement fund was probably eligible to be divided with her anyway, due to the ERISA law, which dictates that retirement accounts are to be divided if the parties divorce. This is without regard to where or not your children are grown or not.
To sum up, I think you would have trouble getting the divorce modified due to circumstances. But, it is possible.
If this is written up with the specifics, and signed by the judge, then if the 20% is not going to the sons as ordered, she could be held in contempt of court for willful disregard of a court order.
She can be looking at jail time. Make sure before you get into this, that the 20% specifically is mentioned to be paid to the boys.