No, there is no wording to guarantee your pension, but there wasn't any wording to guarantee it before either. My personal opinion, however is that the fund will be in much better shape now than it was before for 2 reasons...
1- UPS is paying 6 billion in up front money. The compounding interest of an amount that large will definitely help.
2- UPS is assuming the pension liability of all active employees and Central States will not begin paying it's offset portion until they reach age 65. Until that age, UPS is paying all of the pension. The majority of the drivers I've seen retire are younger than that. That buys CS an additional 5 to 10 years of compounding interest on the money contributed for those drivers before they are required to start paying benefits and that too will help the fund.
I don't know if that makes you feel any better, but I think this plan does more to help current retirees than leaving everyone in the fund would.