Can I Undo a Roth Conversion I Regret?

No — the Tax Cuts and Jobs Act permanently eliminated the ability to "recharacterize" (undo) Roth conversions for tax years 2018 and later. Before 2018, you could convert a traditional IRA to a Roth and then undo it by the extended filing deadline if the account value had dropped. Now, once you convert to a Roth, the transaction is final and the tax is due regardless of what happens to the account value afterward. This makes timing a Roth conversion more consequential — you should be confident in your decision and have the cash to pay the conversion tax from non-IRA funds before proceeding. You can still recharacterize a Roth IRA contribution (changing it to a traditional IRA contribution) — only Roth conversions lost the recharacterization option.