The IRS provided some flexibility in determining first-time eligibility starting in CY2015, knowing that employers would potentially change their look-back periods.
Look-back measurement can be changed as long as the employer maintains the stability periods of the individual employees, whether that was determined from a standard or initial measurement period test.
Employers have to be careful not to fall into a coverage gap. An example would be an employer going from a 6-month to a 12-month measurement period. If an employer had a 6-month measurement period and then switched to a 12-month period that started immediately after the initial 6-month period, there would be a 6-month period where employees in their stability period would continue to be offered coverage until the next test.
The initial measurement period testing months that were in effect when the employee was hired must be used in determining their initial eligibility status.