There is no guidance (formal or informal) on mergers or spinoffs of safe harbor plans. There was a section reserved in the final regs for it, but the IRS has never done anything with it. The best you can do is to use a reasonable good faith interpretation of the existing code and regs.
A new plan would be a successor plan and not eligible for a short initial plan year under 1.401(k)-3(e)(2). Having a final short year for the PEO plan would very likely not be SH under 1.401(k)-3(e)(4). To avoid problems and stay safe harbor, you need to not have two short plan years. Since this is happening mid-year, the only way I know of to do that is to restate the spun-off plan instead of starting a new plan and have the restated plan document mirror the SH provisions used under the PEO plan. The result for participants is exactly the same as it would have been if they stayed in the SH PEO plan. You should be able to change the plan year to end 9/30/19 in the restated plan if you follow the rules in 1.401(k)-3)(e)(3). Others may have different ideas, but that's what I would do.