There are ways of changing it within certain limited situations. For example, certain laws say that if employees sign an agreement to cut back on accrued pensions, it can be done. In some cases, they could say if you haven't earned the complete right to that pension and you're not yet 60 years of age, we can cut it. But the employer would then have to impose cutbacks, and you'd have to find a way to make it palatable to the employees.
It's not a sacred cow. If legislators can change it, that's great. It would be a way to improve the funding stress of the employer's schemes, but it would be at the expense of benefits that go to employees.