Unfortunately with large plans ($1 Billion+), the participants do not have the final say ever since the 2014 pension reform act. I am sure UPS is well aware of this and this is why UPS has not withdrawn from the other pension plans following their exit from Central States. See the link and language below ...
Kline-Miller Multiemployer Pension Reform Act of 2014 FAQs
Different rule for large plans:
There is a different rule for large and financially troubled multiemployer plans (referred to as “systemically important plans”). These are plans that would require PBGC assistance valued at more than $1 billion if the proposed reductions in benefits are not implemented.
Even if a majority of the participants and beneficiaries covered by one of these large and financially troubled plans vote against the proposed benefit reductions, the Treasury Department is required by Congress to permit the implementation of such benefit reductions or a modified version of such reductions.
The Treasury Department, in consultation with PBGC and the Department of Labor, is responsible for formulating any modified version of such reductions, taking into account any recommendations submitted to the Treasury Department by the Participant and Plan Sponsor Advocate.