Can the 2021 tax return be amended for a new plan?

Hi

A one person employer has a 401k plan and for 2021 already put in max deferral and PS and also filed the corporate tax return by 3/15/2022.

Now wants to set up a DB plan for 2021 but take the deduction in 2022 (given the level of 2021 salary, the 31% rule will yield only 9k of DB maximum for 2021 but 2022 will have a high enough cushion to have 2021 and 2022 deduction).

If the corporate return is filed without any extension, too late to amend for 2021 and have a low DB deduction as well as adjusting any other deductions, correct?

If filed with extension, can he go back and redo the deductions (2022 will be an issue as well but easier to deal with).

Thank you

Yes, it is too late. Retroactive adoption under the SECURE Act requires that the plan be adopted before the tax filing due date, including extensions. If the return was filed on time, then there is no extension to the due date - this is true even if an extension request was filed on time.

Thank you and I told them no anyway.

Set up 2022 with traditional DB counting past service.

That is already in the works, thank you.