How do coverage and nondiscrimination tests work for eligibility changes during a year?

Imagine this situation: An employer sponsors and administers a § 401(a) plan that allows § 401(k) elective deferrals, provides matching contributions, and provides nonelective contributions. None of this is a safe-harbor arrangement. All plan, limitation, accounting, and tax years are the calendar year.

When 2022 begins, the plan did not exclude union-represented employees; they were participants under the same conditions as all employees. In the spring, the employer and the union negotiate a collective-bargaining agreement. The CBA, effective June 1, provides for the union-represented employees to be covered only by the union’s multiemployer individual-account (defined-contribution) plan, including for § 401(k) elective deferrals, matching contributions, and nonelective contributions (which all are set to no less than what was provided under the single-employer plan).

Promptly after signing the collective-bargaining agreement, the employer amended its single-employer plan to exclude, from June 1, the union-represented employees. The amendment also specifies that the 2022 nonelective contribution allocated to a union-represented participant is counted only on her January-through-May compensation.

How does a plan’s administrator (and, more practically, its recordkeeper or third-party administrator) run coverage and nondiscrimination tests for this year?

Are there two sets of tests—one for the year’s first five months, and another for the year’s last seven months?

Or are there other ways the measures or tests (or both) adjust for the fact that classifications of participants changed during the year?

Peter, those whose benefits are collectively bargained are mandatorily disaggregated, statutorily excluded from sections of the plan for non-bargaining employees and tested separately, so does it matter?

Thank you. That the collectively-bargained employees are treated as a separate plan and separately tested (even when these employees are covered in a plan the employer could have excluded them from) resolves all the questions I wondered about.