The short answer
Switching payroll providers mid-year does not start a new tax year. Your EIN doesn't change, so nothing resets — not the Social Security wage base, not FUTA, not your state unemployment base, not your employees' W-2s.
The whole job of the migration is teaching the new system about the part of the year that already happened. Load year-to-date figures before the first live payroll on the new provider. Most horror stories trace back to doing those in the wrong order.
What breaks when YTD doesn't carry over
Social Security over-withholding. The wage base is per employee, per employer, per calendar year — $184,500 for 2026. SSA sets it each October for the following year, so confirm the current figure before relying on it. An employee who already crossed the cap under the old provider starts getting 6.2% taken out again.
This one is genuinely painful. If one employer withholds too much Social Security tax, the employee cannot claim the excess as a credit on their return — that credit only exists for people who worked for two or more employers. Same EIN means one employer, so you adjust and repay it, or they're stuck filing Form 843.
FUTA and state unemployment overpayment. FUTA is 6.0% on the first $7,000 of each employee's wages per year, less a credit of up to 5.4% for state unemployment taxes paid in full and on time — a net 0.6% (2025 Form 940 instructions; verify against the current year's form). Employers in credit reduction states get less than the full credit; that list is republished annually in the Schedule A (Form 940) instructions. State unemployment bases and rates are set per state and change annually.
Restart that $7,000 and you pay tax on wages you already covered — and clawing money back from a state agency is far slower than never overpaying.
Additional Medicare Tax timing. You withhold an extra 0.9% once you have paid an employee more than $200,000 in a calendar year, without regard to filing status or what another employer paid. This threshold is statutory and not indexed, so unlike the Social Security base it doesn't move each year. Zeroed-out YTD means the trigger fires late, or never.
Duplicate or split W-2s. Employees should get one W-2 for the year from your EIN, not one from each vendor. Two partial W-2s is a reissue-and-apologize problem in February, the worst month for it.
Switch at a quarter boundary if you can
Form 941 is one return per EIN per quarter. Two payroll companies cannot each file "their half" — a second return for the same EIN and quarter is a duplicate, not a supplement.
Switch mid-quarter and you must decide, in writing, which provider files that quarter's 941 and which only hands over data. The classic failure is both assuming the other has it.
Confirm which form you're on. Some small employers are notified by the IRS to file Form 944 annually instead of quarterly 941s, and you can't switch on your own — a provider that defaults to 941s will file the wrong return.
Switching effective the first day of a quarter makes most of this disappear. Worth waiting three weeks for.
Get these out of the old provider before you cancel
Access dies with the contract, faster than you expect. Pull all of this while you're still a customer:
- The final YTD payroll register, per employee, broken out by gross wages, each tax, each deduction, and each benefit code — not a summary
- Every return filed for the year (941 or 944, state withholding and unemployment) plus deposit confirmations
- State unemployment rate notices and account numbers per state
- W-4s and state withholding certificates
- Garnishment orders with current balances — the most-forgotten item, and stopping a court-ordered deduction is a legal problem, not a payroll problem
- Deduction YTD for anything with an annual per-person cap, especially retirement deferrals and HSA contributions, which reset only at year-end
- Third-party sick pay records
- PDF archives of prior pay statements and quarterly filings
Then handle authorizations: a new reporting agent needs Form 8655, and each state agency needs its own third-party access update.
Also know which kind of vendor you had. A reporting agent or payroll service provider files on your behalf, but you remain liable if it defaults — that deposit is still yours. A certified PEO is a different arrangement entirely.
The reconciliation that catches everything
Reconcile at two levels before you sign off.
In aggregate: your four quarterly 941s (or the single 944) should tie to your W-3 totals: wages, federal withholding, Social Security and Medicare wages and tax. Form 941 is an aggregate return; that's the level it reconciles at.
Per employee: each W-2 should tie to the payroll register, old and new provider combined. Check your highest-paid employee against the wage base first — a bad import shows up there largest.
Do this in October or November, not January. A wage-base error found in Q4 is a payroll adjustment. The same error in February is a Form 941-X, a W-2c, and a conversation with an employee about their tax return.
If it already went wrong
Corrections exist and they work. Over- or under-reported employment taxes get fixed on Form 941-X; already-issued W-2s on Form W-2c. Over-withheld Social Security should be repaid to the employee by you and adjusted on your return; Form 843 is their fallback only if you don't.
One distinction worth knowing: the predecessor/successor rules in Rev. Proc. 2004-53 apply when one business actually acquires another and takes on its employees. A vendor change is not an acquisition — same EIN, same employer, one continuous year. Don't let anyone talk you into treating it like a corporate transaction.
Keep your own copy of the record set
Recordkeeping fractures during a migration: some pay statements sit in the old portal, some in the new, and access to the old one ends with the contract. Hold your own copies of every statement and filing for the year — not links into someone else's system.
UWageCo generates pay statements and business documents from register data you supply, one way to keep that record set consistent across a vendor change. Be precise about what that is and isn't: documents come from data you provide, each carries a disclosure saying so, and nothing is independently verified or endorsed. Preparing a document is not filing one — the 941s, 944s, and W-2s remain yours or your accountant's to file.
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*Not legal or tax advice. Wage bases, rates, and state rules change annually and vary by state. Check current IRS and state agency publications for your filing year, or talk to a CPA about your situation.*