Of course, the best time to make the switch is when starting a new benefit year. If you're switching mid-year and you're accruing benefits based on an annual award, then you can just enter the current employee balances as carryover into Virtual TimeClock. You'll just need to adjust the carryover based on your business rules at the end of the benefit year and set up the new annual award amounts.
If you're already accruing benefits based on a calculated award, then you can still enter current employee balances as carryover into Virtual TimeClock, but then you'll need to either adjust the calculation rate based on the time remaining in the current benefit year, or set the employee's award anniversary to the date you want to start using your employee time clock software to start tracking leave. You'll still need to adjust the carryover and accrual rate at the end of the benefit year.