Bill Presson gives you a helpful description of Internal Revenue Code § 410(b)(6)(C)(ii)’s transition period with your stated assumption that “the change in members of a group” happened in 2023 and an embedded assumption that all plans’ years are calendar years.
If a purchase closed on January 1, 2024 (rather than, for example, on December 31, 2023), that might result in a different § 410(b)(6)(C)(ii) transition period.
Lawyers and accountants in deal teams sometimes consider accounting, banking, insurance, securities, employee-benefits, tax, and other consequences that might follow from exactly when a transaction closes. A deal some businesspeople think of as a December deal might have a transfer of ownership timed for January 1.
A practitioner might check the details.