"A retirement plan will not be treated as having a 'partial termination' during any plan year that includes the period beginning on March 13, 2020, and ending on March 31, 2021, if the number of active participants on March 31, 2021, is at least 80% of the number of active participants on March 13, 2020.... Plan sponsors may be able to meet the March 31, 2021, threshold by increasing plan participation in ways beyond just hiring or rehiring employees, such as through expanding eligibility to part-time employees or other excluded classes."