ERISA requires that a retirement plan must be 100% nonforfeitable as to contribtuions within 6 years (7 years for multiemployer plans). 29 U.S.C. 1053. Based upon this requirement, to the extent that the change of benefit can be shown to undermine your and your employer's prior contributions, then the fact that you did or did not receive notice of a change in benefits is irrelevant. What is relevant is whether or not your benefits have been undermined, because if so, then that act is a breach of trust and a violation of ERISA. 29 U.S.C. 1105.
You can file a complaint with the U.S. Department of Labor -- Employee Benefits Securities Administration, at: 1.866.444.EBSA (3272).
Hope this helps.
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