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There is no guidance, either formal or informal, regarding mergers or spinoffs of safe harbor plans. There was a section for it in the 2006 final regs, but it only said "reserved". In absence of guidance, a reasonable, good faith interpretation of the code is your best option.
It would be helpful to know your role in this. It may affect the advice you will get from others. You'll also want to keep in mind that whatever route they take, they will need cooperation from the new service providers and probably the old ones, too. If the new service providers don't have someone qualified to assist them, the plan sponsors may want to reconsider their choices.
We have, in the past, handled similar situations and continued the safe harbor for the participants for the entire plan year, with the understanding that we might have to justify it to the IRS at some point. That's a judgment call and not everyone will make the same decision. Hopefully, we can get a discussion going.