Retirement savers, give thanks! The recently passed tax plan doesn't harm you - much. Congress, for instance, did not lower maximum contributions for tax-deferred plans, like traditional 401(k)s and individual retirement accounts. Nor did Congress tinker with moving your money from a traditional plan into a Roth, where you pay the taxes up front and appreciation grows tax-free and your withdrawals won't ever be taxed.
With one small exception. Until Congress changed the law right before the holidays, some people who did a Roth conversion could decide they wanted to un-do it, which the tax experts call a recharacterization. Not anymore.
Backing out of a Roth conversion was a convenient deal. Maybe at year's end, you discovered that other unanticipated deductions, income or market conditions made the conversion look like a boneheaded idea. Maybe it turned out you lacked the money you thought you'd have to pay the Roth tax. Maybe the stock mutual fund you converted had slid since you moved the fund into a Roth IRA. Why pay taxes on higher-cost stock when you can make the conversion disappear - and maybe later convert at the lower level?
Under the new law, for tax year 2018, you can't have that flexibility anymore. Once you make the switch, you are stuck with your choice. The good news: if you want to recharacterize a 2017 Roth conversion, you have until Oct. 15, 2018 to do it.