You can always be more generous than the law requires, if the plan document is amended to so provide. Otherwise, for participants with less than 3 years of service as of the date of plan merger, those participants have to be given a vesting percentage no less than their existing vested percentage as of the date of the merger. Thereafter, they could be subjected to the surviving plan's vesting schedule even if the predecessor plan's schedule was more generous at a later point. As applied to participants with 2 years of service under Plan A, such participants have to be at least 30% vested. After 3 years of services, and so on, their vested percentage would be determined applying the provisions of Plan B's vesting schedule, unless the plan is otherwise amended to either continue the Plan A schedule to their Plan A accounts or their vested percentage is otherwise increased under Plan B.