A payroll conversion is the highest risk routine change most organisations make, because the failure is immediate, visible and personal: people are paid the wrong amount. The risk is concentrated in one place, the carrying forward of year to date balances, and it is entirely manageable with a deliberate sequence.
Year to date balances are the whole risk
Every subsequent calculation and filing inherits them. Get them wrong and the errors are not visible in the first run but appear at thresholds and at year end. Reconcile the balances before go live, agree them with whoever is leaving, and keep the evidence, because reconstructing them afterwards is considerably harder.
Choose the timing deliberately
A tax year boundary is cleanest because balances start from zero. A quarter boundary is the next best. A mid quarter conversion means two sets of figures to combine at year end, which is workable and adds a reconciliation nobody enjoys. Move the date rather than accept a rushed cutover.
Run in parallel, twice if you can
One parallel period proves the configuration; a second proves the exceptions. Compare every figure rather than the totals, because offsetting errors produce a matching total and two wrong payslips. Where differences appear, explain each one rather than accepting that the new system is simply calculating differently.
Questions people ask about payroll conversion
When is the best time to convert?
The start of a tax year, because balances start from zero. Anything else adds a reconciliation at year end.
How many parallel runs?
At least one, preferably two, and the second should deliberately contain awkward cases rather than a clean month.
Who checks the balances?
Somebody from both sides, with the result agreed in writing. This is the handover that matters most and is often the least formal.