It's not really much of an answer to say that the government took some people as employees without realising how long people would live. Don't think you or I would be able to novate a contract simply because it turned out to be more burdensome than we expected, hoped, or predicted, and yet that is what the government is doing; purporting to substitute one contract with a new one, is it not?
Where in the contract does it say that an employee who starts with a pension in one form, an essential term in the contract, part of the consideration for it, is liable to have that pension changed according to the whim or wish of one party to it ?
If he proposed change applies only to those who become employed after the change, then there's no problem; they either take as the new contract is or refuse it, but that does not appear to be the case here.