Does the 2020 W-4 form change impact prior year existing IRS lock-in letters for employees?
No - Continue to use the same withholding restrictions for an existing lock-in letter unless you receive a modification notice from the IRS. However, if the employee provides a new W-4 (2020) that provides for more withholding than is the case under the lock-in letter, you must honor the new W-4. See Page 23 or the 2020 Publication 15 - Employer's tax guide.
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