To avoid tax on this conversion, you need to file or had already filed a Form 8606 showing a non-deductible contribution. See Form 8606 Lines 1,2.
OR
If you took a deduction for the traditional IRA contribution, that offsets the tax, so it all balances out and there is no reason to amend.
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Amend with a properly filled out Form 8606 attached and some or all of your tax on that conversion will be refunded.
NOTE: Generally, if you already had a balance in IRAs before attempting this, you cannot do it completely tax-free.
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