“I have been divorced for 3 years, my ex-wife contacted me and said that she was going to go back to court to try and get 1/2 my 401 K and pension. In the divorce agreement it states that I retain all rights to this. Can she reopen this after 3 years?”
In California, the answer is “yes, your ex can reopen your divorce proceedings three years after to divide an asset.” If, as you say, the 401(k) has already been divided or accounted for completely in the judgment, then she will likely be unsuccessful. The Court will probably hear it though.
Family Code Section 2556 provides: In a proceeding for dissolution of marriage, for nullity of marriage, or for legal separation of the parties, the court has continuing jurisdiction to award community estate assets or community estate liabilities to the parties that have not been previously adjudicated by a judgment in the proceeding. A party may file a postjudgment motion or order to show cause in the proceeding in order to obtain adjudication of any community estate asset or liability omitted or not adjudicated by the judgment. In these cases, the court shall equally divide the omitted or unadjudicated community estate asset or liability, unless the court finds upon good cause shown that the interests of justice require an unequal division of the asset or liability.
So this means for you that the Court restains the authority to divide any undivided property. If the 401(k) was properly disclosed, an accurate value given, and an equal division of assets accomplished while taking into account the value of the 401(k), then you are in a good position. If not, she will likely get one half of the contributions attributable to marital efforts.
You may be saying, but “geez 3 years, isn’t that too long to wait? What about the principal of ‘ya snooze, ya lose?'” At least in California, laches (unreasonable delay or snoozing) would not apply unless there was actual prejudice to you. If your argument to prevent the division was based solely on the delay, the Court would hear the case and divide the asset unless it had been otherwise accounted for equitably in the judgment.