Transition from the standard 35 hour workweek to working duration in days/year

We are currently using Working Hours frequency as Weekly (for example, 40 hours per week). Now customer wants to move to 'annual days package' where a system in employment contracts that allows for a departure from the standard 35-hour workweek by counting work duration in days over the course of the year instead of hours per week. This system offers more flexibility for employees in organizing their schedules.

Please provide guidance on steps to do this and implications on FTE calculation.

Thanks

Kishore