In most cases your pension is protected by the Pension Benefit Guaranty Corporation, a Government Agency specifically for that purpose. Think of it as the FDIC of pension plans, as it is insurance that pays monthly benefits up to a guaranteed maximum should our plans fail or become insolvent.
You can search here to see if your multi or single employer plan is covered.
Pension Benefit Guaranty Corp - PBGC Protects America's Pensions
Western Conference of Teamsters Pension plan is covered for fellow Teamsters in my area.
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Should UPS seek to end participation in the pension plans as they are now, it would be something negotiated in a contract year. Using WCT as an example, what was contributed would stay if you were vested and be paid at the previous terms & whatever was negotiated would fill in from that point (the same as if you quit or were fired your benefit freezes and is paid at 65 or PEER80 if you have 25 years in). As you can imagine, this would cause major issues and when it was tried in 1997 it led to a general strike. I wouldn't lose any sleep about that happening again, even in this generally anti-pension climate.