Nope, the details when you retire can and have changed retroactively for retirees. It is a defined pension plan so you will never get more than promised when you retire (no cost of living or later increases) but your benefits can be reduced. Already retirees got hit with healthcare increases above what was promised when they retired.
The CS health benefits are and have been separate from pension benefits and entirely a Central States health plan without lawful guidance.
By ERISA law, defined pension benefits couldn't be reduced either until the MPRA passed 12/2014 that allows reductions to underfunded multiemployer plans only ... for now.
Perhaps this doesn't change anything, but this defined benefit is offered under a UPS/IBT single employer plan now rather than CS multiemployer. One legal requirement for exiting CSPF was UPS had to set up another pension fund for
active members. The dilemma of service & payout from both funds were considered. It was set up to include defined benefits for varying levels of service & accrual credits with the language that UPS offsets CS deficits. This would be the UPS/IBT Full Time Pension Plan which was allowed to set up with "offsets" a condition of the plan & CS withdrawal; which of course the trustees can attempt to change for no reason whatsoever!