Company Wants to Leave PEO Plan

I have a prospect that is currently engaged with a PEO, but unhappy with it.  What they would like to do is start a new 401(k) (along with other benefits) for their office staff immediately, then at year end, when the contract with the PEO runs out, move all the warehouse/PEO employees into the new 401(k) plan.  It would seem to me that it's possible to do this, but I feel we may need to test the plan as a whole, including all the employees currently in the PEO plan?  The plans would have the same benefits, eligibility, match, etc. so that's not an issue.  Would it be possible to exclude leased employees (those under the PEO) from the new plan and then once they become true employees of the new company at year-end, they would become eligible for the independent plan?  Odd circumstances, I'm not sure exactly why they would want to do it this way, it seems needlessly complicated.  They described it as wanting to use the office employees as the guinea pigs for the new benefits so that when the PEO employees join the plan there wouldn't be any hiccups.  Any thoughts are appreciated.