Big severance "carrot" taken away

My wife worked for a company 19 years. She had stellar performance throughout her career and was the corporate secretary for her last 5 years. She was on the company's lucrative supplemental benefit plan which provided substantial benefits to her if a change in control occurred. During the fall of 2000, the CEO of the company told her she would no longer be the corporate secretary and that the supplemental plan was being reworked.
A less qualified male was put in her position. She was told she was being painted in a corner because she may have indicated her desires to stay at home with our children. She was told she could stay on working but in a lesser capacity. She never gave a resignation until the CEO requested it and her last day was Nov. 30, 2000.
She did manage to get a one year severance payment but never signed any type of document normally associated with a severance. She was gone due to the "big severance carrot" being taken away. Well it turns out shortly after her departure, a deal was in the works to sell the company. The deal was annouced in June, 2001 and completed in January, 2002. The other executives which were on the old supplemental plan were now all on employment contracts and got payments equalling 3 years salary plus other benefits. Does she have any case here or is it too late to sue?