I'm guessing the article is a bit imprecise. Most pensions have some benefits which are vested and some which are not vested, usually determined by time in plan. Benefits which are vested would be guaranteed, by the government (funded by pension providers) if not by the company, and the employee would not lose any pension credits already earned. MLB could, however, stop issuing new credits to those already in the plan, i.e. they wouldn't lose the pension they have but the amount of the benefit would not increase any further from this point on, no matter how much longer they worked.
Even so, thats not what they were promised when they were hired.
Even if they get to keep what is vested, we all know that most often these pension plans are weighted heavily towards the back end of the vesting period.
Maybe MLB is different, but the company my brother in law works for, AND the company my wife works for (2 completely different companies) both have pension plans that vest drastically more rapidly in the later years, than the early years.
The reason you stay at this company for the first 10 or so years is a sacrifice for the next 10 or so years. But to suddenly pull the rug out after those first 10 years and tell an employee, "well, you held up your end of the bargain, but hey,we've voted to not hold up our end of it", seems very wrong and greedy to me.
If they want to change the policy for any new hires going forward, that is one thing, but they shouldnt be changing or altering what the current employees were promised when they were hired.