Payroll System Migration UAT Documentation
Migrating to a new payroll system requires proving, before go-live, that the new system calculates pay correctly, usually through parallel runs where the same pay period is processed in both the old and new system and the results compared line by line, and doing that comparison manually across even a modest employee population, checking gross pay, every deduction, every tax calculation, net pay, is slow enough that testing teams under go-live deadline pressure often narrow the sample or accept a discrepancy as 'probably a rounding difference' without actually tracing its cause. A discrepancy dismissed during UAT that turns out to be a genuine configuration error in the new system becomes a live payroll error affecting every future pay run once the old system is switched off.
STARTING PRICE
From €799
Complex tier · Multi-system orchestration, custom logic, and higher-volume or higher-risk processing.
Get a quote →Saves roughly 20-40 hrs across a migration's UAT cycle for a mid-sized employee population, plus materially reduced go-live risk.
How the automation works
We run the line-by-line comparison between old and new system outputs for every parallel test cycle, gross pay, every deduction category, tax calculations, net pay, for the full test population rather than a sample, and every discrepancy is logged with its specific source, not accepted as an assumed rounding difference until it's actually traced to one. Each discrepancy is categorized, a genuine calculation error in the new system's configuration, a legitimate difference from an intentional process change, or an actual rounding difference within an acceptable tolerance, and go-live readiness reporting shows exactly what's been resolved, what's still open, and what's explicitly accepted as a known, documented difference, giving whoever signs off on go-live real evidence rather than a general sense that testing went fine.
Process flow
- 01
Parallel test cycle run trigger
The same pay period is processed in both the legacy and new payroll system for the full test population.
- 02
Compare line by line ai
Gross pay, every deduction category, tax calculations, and net pay are compared line by line between the two system outputs for every employee in the test.
- 03
Categorize every discrepancy ai
Each discrepancy is categorized as a genuine calculation error, a legitimate intentional process difference, or a rounding difference within accepted tolerance, with its source traced, not assumed.
- 04
Route errors for resolution output
Genuine calculation errors route to the implementation team for configuration correction, with the discrepancy re-tested in the next parallel cycle.
- 05
Deliver go-live readiness report output
A readiness report shows resolved, open, and explicitly accepted discrepancies, giving the go-live decision-maker documented evidence rather than a general testing summary.
Inputs
- Legacy system payroll output for test periods
- New system payroll output for the same test periods
- Test employee population and pay scenarios
- Discrepancy tolerance and acceptance criteria
Outputs
- Line-by-line comparison per employee per test cycle
- Categorized, traced discrepancy log
- Resolution tracking for genuine errors
- Go-live readiness report with documented evidence
Works with
Prefer a fully custom build instead of an off-the-shelf integration? We scope both options during your free consultation — most jobs like this one work fine on standard connectors, but higher-volume or non-standard systems sometimes need bespoke API work, reflected in the complex tier.
Where this goes wrong if you get it wrong
- A discrepancy dismissed as 'probably rounding' without actually being traced to confirm it's a rounding difference, rather than a genuine configuration error that happens to be a small dollar amount, is how a real calculation bug slips through UAT and becomes a live payroll error once the old system is decommissioned and there's no parallel comparison left to catch it.
- Testing only a small representative sample of employees rather than the full population, or the full population but only for a straightforward standard scenario, misses edge cases, an employee with multiple garnishments, a part-year hire, a multi-state work pattern, that are exactly where a new system's configuration is most likely to have a genuine gap.
- Under go-live deadline pressure, accepting an unresolved discrepancy as 'we'll fix it after go-live' converts a testable, low-stakes UAT finding into a live production payroll error affecting real paychecks, discrepancies genuinely need to be resolved and re-tested before go-live, not deferred because the deadline is close.
- A go-live sign-off based on a general sense that 'testing went well' rather than a documented report showing specifically what was compared, what discrepancies were found, and how each was resolved or explicitly accepted, gives whoever approves go-live no real basis for that decision, and no record to point to afterward if a problem emerges.
Frequently asked questions
Does this test the full employee population or a sample?
The comparison runs across the full test population you define for each parallel cycle, specifically because edge cases, unusual pay scenarios, multiple garnishments, part-year hires, are where genuine configuration errors are most likely to hide.
How are discrepancies distinguished from expected rounding differences?
Every discrepancy is traced to its actual source and categorized, a genuine error, an intentional process difference, or a true rounding difference within tolerance, rather than assumed to be rounding without verification.
What does the go-live readiness report actually show?
Specifically which discrepancies were found, how each was resolved or categorized, and what remains open, giving the sign-off decision documented evidence rather than a general summary.
Can this be used for a phased migration across multiple entities or countries?
Yes, parallel comparison and discrepancy tracking work per test cycle, so a phased migration can run this process for each entity or country as it comes up for cutover.