I have worked for a major aerospace company since 1977. When I started, the company offered an 85-point retirement. That means that I can get full retirement benefits [about 50% of current pay] when I reach 85 points. Points are defined as age plus years of service (rounded up). Benefits are considered "earned" starting at about 5 years of service. No one can collect benefits before age 55.
The effect is that earned retirement doubles overnight as you reach 85 points.
In the 1980's, the company decided that this was too lucrative, so they changed the deal to some other plan. Having some morals [and trying to avoid lawsuits], they grandfathered the old plan for current employees [me] and applied the new plan to new employees.
Around this time the retirement fund was doing really well. Large government contracts were funding big bucks for the plan, interest rates were high and any fool could get a good return on investments. The board of directors decided that they could take "excess' money out of the plan, since they put it in, and record it as profit [paying tax on it.] This was quasi-legal at the time. Shortly thereafter, Congress figured out that this practice greatly increased the risk of future bankruptcy of retirement plans, which the public coffers would have to pay for. Congress passed legislation to make this practice illegal. While this law did not stop companies from looting the retirement plans, it did make it much, much harder. Now, they must generally terminate a plan to get the money out.
As time went on, the company was split up. Parts were sold to other companies, most parts were spun off into new stand alone companies. The retirement plans were split up by some arcane formula. Retirees and potential retirees were assigned to companies. [My wife, who used to work for the company, will draw a small retirement from Boeing when she reaches age 55. She has never worked for Boeing, who bought part of the company, a day in her life.]
Over the years, the company has made several changes to the retirement plans. They used to offer full medical benefits to retirees. Now the premiums to be paid by the company have been capped. They have slowed the rate of retirement "earnings". They have always been careful to state that the plan can be changed at any time. When they did the final spin off of the current company, they stated that they would only protect the current plan through FY2003, which ends with September.