That is considered a "conversion". It will be fully taxable at your regular tax rate. If you do the conversion before 12/31/25, you will need to make an estimated tax payment by January 15, 2026, AND you will need to include a penalty calculation form on your tax return (form 2210 with schedule AI, the annualized income method) that will show the IRS that your income was not evenly spread over the year, but you made appropriate tax payments when needed. Otherwise, you might be assessed an underpayment penalty.
However, if you do the conversion between January 1 and March 31, 2026, you can spread out your estimated payments over 4 payments due April 15, June 15, Sept 15, and Jan 15, 2027.
You can also do a partial conversion if you don't want to pay all the tax at once. You can do a tax-free rollover from the 401k to a pre-tax traditional IRA, and then do partial conversions over time as you can afford the taxes.
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