New 401k Plan?

Can New Company start a 401k plan effective 1/1/24?
New Company with new EIN - owned 100% by one of the partners of Old Company.
Old Company - owned 50/50 with a partner. They split up and dissolved the company (12/1/23) and each started their own company. Employees went with them mostly. Old Company had a 401kPS and DB plan. Both plans are terminated (December 2023) but not fully paid out yet.

If your concern is the rule that restricts the adoption of an alternative defined contribution plan (Treas. Reg. §1.401(k)-1(d)(4)), that rule only apply to plans successor plans adopted by the same employer.  In the facts you provide, I assume New Company is a new company, so that isn't an issue.

If your question is whether you can start a new 401(k) plan in April 2024 that is effective on the first day of 2024 - sort of.  You can start the plan now, the non-deferral portion can be retroactive to the beginning of the year, but deferrals can not be made retroactively - so you want to make sure the deferral provisions are effective when the plan is actually implemented.

Yes, my concern is whether New Company is technically the same employer as Old Company in spite of the change in ownership and EIN. He owned 50% of the previous company and all but one of his employees came with him from Old Company.

Even when that alternative-plan rule applies, it might not preclude a new organization from creating a retirement plan, even one that includes a § 401(k) arrangement.

Rather, the consequences fall on the “old” plan. That plan might have paid a too-soon distribution—absent some circumstance (perhaps including age 59½) that under the “old” plan’s provisions allows a distribution from the participant’s elective-deferrals subaccount. That plan’s supposed cash-or-deferred arrangement might be treated as not a § 401(k) arrangement. And that plan might be tax-disqualified if, in approving a too-soon distribution, the plan’s administrator acted contrary to the plan’s written provisions. Further, a too-soon distribution from a tax-disqualified plan might not be an eligible rollover distribution.

But none of those consequences by itself precludes a new organization from creating a retirement plan, even one that includes a § 401(k) arrangement.

The challenges are about administering the “old” plan.

And the new organization’s plan might refuse an attempted rollover contribution if the would-be-receiving plan’s administrator knows the distribution from the “old” plan was not an eligible rollover distribution.

26 C.F.R. § 1.401(k)-1(d)(4)(i) https://www.ecfr.gov/current/title-26/part-1/section-1.401(k)-1#p-1.401(k)-1(d)(4)(i).

This is not advice to anyone.