Simple plan and profit sharing plan

Hi, we have a client with a Simple plan that is considering replacing with a safe harbor 401(k) plan.  We *think* that's not possible for 2025, because we would've needed to terminate and replace by 10/1 - and it's too late for a 30 day notice.  Are we thinking correctly?

Alternate option we were thinking is to open a profit sharing only for 2025.  We are getting conflicting information on whether you can have a PS only at the same time as the simple.  We searched online and came up with this, but aren't sure of the accuracy: Yes — allowed, but operationally awkward. Carefully decide whether to (A) keep the SIMPLE and add a separate profit‑sharing plan (two plans to administer) or (B) terminate the SIMPLE mid‑year and adopt a consolidated 401(k)+profit‑sharing plan (requires 30‑day SIMPLE termination notice and adherence to SECURE 2.0/IRS Notice 2024‑02 procedures).   

thanks!

A SIMPLE IRA has to be the only plan sponsored by the employer. 
 

https://www.irs.gov/retirement-plans/retirement-plans-faqs-regarding-simple-ira-plans

The only exception to the exclusive plan rule for a SIMPLE is a safe harbor 401(k) plan. The rule required the SIMPLE to end and the safe harbor 401(k) deferrals to begin the next day and the deferral limits are then pro-rated for the year.

I agree that the safe harbor 401(k) plan must have at least 3 months of deferrals in the year to allow the plan to be safe harbor, so you may likely be out of time for 2025.

Here is a thread TL;DR