What breaks when a client switches payroll providers mid-year

A client moves to your firm in June. The old provider hands over a report, you load the employees, and the first payroll runs clean. Nothing looks wrong — and that is the problem. The errors from a mid-year switch do not appear in the first payroll. They appear in January, on a W-2.

Annual caps belong to the employee, not to the provider

Social Security stops at a wage base each year — $184,500 for 2026. FUTA applies to the first $7,000 per employee. State unemployment has its own base. All three are per employee per calendar year, and none of them restarts because the software did.

A system that begins counting at zero in June will keep withholding Social Security past the cap and keep charging FUTA on wages already taxed. The employee is over-withheld; the employer overpays; and the W-2 that comes out in January is short the wages the previous provider paid.

The reason is plain in Publication 15: a payroll service is an agent, not the employer. “A PSP performs these functions using the EIN of the employer… the employer remains liable for its employment tax obligations.” Same employer, same EIN, same year: one running total.

Changing providers is not an acquisition

There is a real rule for wage bases carrying over, and it is about buying a business, not changing software. Publication 15: “When corporate acquisitions meet certain requirements, wages paid by the predecessor are treated as if paid by the successor for purposes of applying the social security wage base limit…” with the worked example of an employee already paid $2,000 — “subject to social security tax on the first $182,500 ($184,500 minus $2,000).” FUTA has its matching successor-employer provision.

If your client simply switched vendors, none of that applies, because there was no predecessor employer. There is one employer that has been paying wages all year.

One W-2 or two?

This is where firms most often guess. For a genuine acquisition, Rev. Proc. 2004-53 gives two options. Under the standard procedure (Section 4), each party reports what it paid and the employee receives two W-2s. Under the alternate procedure (Section 5), the successor issues one combined W-2that includes “wages paid, and taxes withheld, by both the predecessor and the successor,” and the predecessor is relieved of filing Copy A for those employees.

For a provider change with no change of employer, neither procedure is in play: the EIN is unchanged, and the employee gets one W-2 for the year covering all of it. Which means somebody has to carry the prior numbers forward — and that somebody is the system running payroll in December.

The failure mode to avoid: both the old provider and the new one issue a W-2 under the same EIN. The SSA receives two partial forms for one person and flags the mismatch.

What to get from the outgoing provider

• Year-to-date gross wages per employee, through the last check date they processed.
• Federal income tax withheld, Social Security and Medicare wages and taxes, separately.
• State wages and withholding, per state.
• FUTA and state unemployment taxable wages per employee — not just the tax paid.
• Copies of every return already filed for the year, and the deposits behind them.
• A written answer on whether they will issue W-2s for their portion.

Ask for the figures rather than estimating them from a pay stub. Wage bases are the one place where an approximation is indistinguishable from a correct number until the year closes.

PayrollPro records what the previous provider already paid, per employee, and applies it to every cap and to the W-2 — and it refuses to guess the one decision that is not ours: who issues the consolidated form.