People who save for retirement in a traditional IRA or 401(k) are often met with an unwanted surprise -- required minimum distributions, or RMDs.

RMDs aren't always a problem, especially when they're on the smaller side or represent funds that were going to be withdrawn anyway. If you're on the hook for a $12,000 RMD but you need $1,000 per month from your savings to supplement your Social Security checks, that mandatory withdrawal isn't such a blow.

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Rather, RMDs are an issue when they're large and/or represent money you don't need. That's because they could trigger a huge tax bill and other unwanted consequences, like surcharges on your Medicare premiums.

You'll often hear that if you want to avoid RMDs, a Roth conversion is a great way to get out of them. But one thing you must realize is that a Roth conversion is a taxable event. And if you don't plan for it carefully, it could end up backfiring on you.

Why Roth conversions can backfire

With a Roth conversion, you move funds from a traditional retirement account to a Roth IRA. You then owe the IRS money on that conversion the year you make it.

This means that if you do a large Roth conversion in a single year, your tax bill could be huge, especially if it pushes you into a much higher tax bracket than usual. But that doesn't necessarily just mean owing the IRS more money.

Just as large RMDs could lead to surcharges on Medicare premiums, so too could a high-income year from a Roth conversion. That's why it's important to plan for that conversion carefully and, ideally, spread it out over multiple years.

First, try to figure out when you might have some lower-income years and how many. From there, it's a matter of division.

Say you expect to retire at 63 and live off of dividends and capital gains from a taxable investment account for four years before claiming Social Security. That four-year window may be a prime one for a Roth conversion.

If you have a $600,000 balance, moving $150,000 a year into a Roth IRA could lead to a much lower tax bill overall than doing that conversion in a single year, or over two years.

Plan ahead for better results

A Roth conversion isn't something to jump into. It's important to plan for yours carefully so it ends up serving its purpose -- saving you money on taxes and helping you avoid the less obvious but undesirable consequences that come with having a higher taxable income.