Excluded Employee Participating in DC Plan

[Guest ralar]

A plan document is (mistakenly) amended to exclude union employees from participating in the plan. Only one union employee was participating at the time of the amendment and his status was not changed to "ineligible." Therefore he kept participating.

Under Rev. Proc. 2001-17, when an ineligible employee (due to minimum age and service requirements) participates, the plan can be retroactively amended to change the age and service requirements so that the employee (and all like situated employees) can participate.

Can the plan also be amended when the employee who participated is a member of an excluded class (i.e., retroactively amend the plan to allow union employees to participate)?