Plan to Plan transfers

[Guest soccermom]

An employer (Plan A) who has union employees is interested in moving its 401(k) plan into a large multiemployer 401(k) plan (Plan B). Where are the rules for a plan-to-plan (AKA trust-to-trust) transfer found? Do participants have a choice as to whether they stay with Plan A or move to Plan B? What are the mechanisms for accomplishing this? Does Plan B have to set up the accounts in the person's name ahead of time? How long can the black-out period be?

To make matters more complicated Plan A has a few members in it that participated in a prior Money Purchase Plan which offered J&S. Plan B does not offer J&S. Doesn't Plan B have to continue to offer that choice to those Plan A participants who had this benefits right and feature before?