Payroll change from Lunar to Calendar

 Hi, I have a customer who runs a lunar payroll in the GB;  they have a handful of employees that need to move from lunar payroll to calandar payroll;  main reason is to be able to perform a quick pay calculation as per calendar and is purely for reference;  they will not acually process this set of employees thru the payroll processes (overseas employees on calendar month, but want to calculate value of GB tax and NI).

is there a preferred way to to this?  are there any gotchas that need to be considered that people are aware of?

As the employees are only processed this way for reference, I'm sensing the biggest decision is decision when from and when to perform the update....