ACK

As the EIN of the purchased company is being changed, my guess is that this was a stock sale and the purchaser is changing the EIN. If it was a stock sale, then the purchaser has assumed the SIMPLE plan and must maintain it until the end of the year. The employees of the acquired company will continue to participate in the SIMPLE and the purchaser must fund the necessary employer contribution until year-end . At that time, the SIMPLE may be terminated and the employees of the acquired business will be eligible for the purchasing company's plan based on the eligibility requirements of that plan. The acquired employees cannot participate in the both the SIMPLE and the purchasing company's plan. Only the employees of the acquired company may participate in the SIMPLE and the termination can only be done as of 12/31.

In response to the original question of whether a retroactive corrective amendment can be made, it cannot be made under SCP. There are only a few reasons for corrective amendment via SCP, as discussed here, and I don't believe this situation satisfies any of these. https://www.irs.gov/retirement-plans/fixing-common-plan-mistakes-using-a-plan-amendment-for-correction-in-the-self-correction-program Otherwise, the corrective amendment requires a filing.

I don't know how the DB world works, but when I have this situation with a DC plan, if both spouses do want to be covered under the same plan, I still need to have a conversation with them about children, involvement, etc. Those things do matter because if there is a control group, then the plan is a single employer plan. If there is not a control group, then it is a multiple employer plan. So assuming both spouses want to be under one plan, I need to know whether I am indicating in the document that it is single employer or multiple employer and the 5500 is filed differently, depending on what type of plan it is.