Hi,
It looks like, your client wants to go-live in april (which is not mid-year go-live).
You can make a copy of Production system to quality/or any other temporary system and they can check the correctness of the data.
Because, the moment you say that other modules are live, it may create confusion, if you run payroll in production system. Also, you may have to find a strategy for avoiding payroll posting in production system, because, the moment you start running payroll, it will generate posting results (which you say, it is through legacy).
regards,
Bala.