At 68, He Plans to Sell a Long-Held S&P 500 Fund and Shift to Bonds. The $310,000 Gain Could Lift His Medicare Premiums in 2028

Turning a decades-old index fund into bonds may seem like a simple retirement adjustment, but a large capital gain can quietly trigger a Medicare surcharge that appears two years later and costs far more than many retirees expect.

Picture a hypothetical 68-year-old who bought an S&P 500 index fund in a taxable brokerage account in 1996. He now wants less exposure to stocks and plans to sell the fund and move the money into bonds. With the 10-year Treasury yielding 4.91%, the change seems reasonable. For context, the SPDR S&P 500 ETF Trust (NYSEARCA:SPY) has climbed 466.8% in price since November 1999 alone.

The sale produces a $310,000 gain. He has accounted for the capital gains tax, but the Medicare charge that arrives two years later has not occurred to him.

The Medicare income-related monthly adjustment amount, or IRMAA, affects about 8% of Part B enrollees. A single filer with MAGI of $109,000 or less ($218,000 for married couples filing jointly) pays the standard $202.90 per month. Most retirees remain below that threshold. One sizable sale can push a retiree with otherwise moderate income over the line for a year.

Why Selling a Fund Can Affect Medicare Income

Social Security uses the most recent federal tax return information provided by the IRS, which is usually the return from two years earlier. A sale in one year sets the premium two years later. SSA defines MAGI as adjusted gross income plus tax-exempt interest income.

The account type matters. Trades inside a traditional IRA or Roth IRA do not appear on Form 1040, so swapping stocks for bonds there does not add to MAGI. A sale in a taxable brokerage account, by contrast, sends the gain to Schedule D and directly into AGI.

It is important to get the basis right before selling. Every reinvested dividend was taxed in the year it was paid, and each one increased the cost basis. Three decades of reinvestments can materially reduce the real gain. The $310,000 figure here assumes those dividends are already included.

A $310,000 Gain on Top of $80,000 in Income

Assume he files as a single taxpayer and has $80,000 in other MAGI. Without the sale, he would pay the standard premium. The gain lifts his MAGI to $390,000, placing him in the single-filer bracket above $205,000 but below $500,000. Using 2026 rates, Part B rises to $649.20 per month, which includes a $446.30 surcharge. If he also has Part D coverage, an $83.30 monthly surcharge applies as well. Together, those surcharges total $6,355.20 a year. Actual 2028 premiums have not yet been announced.

A married couple filing jointly at the same income would each pay $527.50 for Part B, and both spouses would owe it. After one spouse dies, the survivor may later fall under the lower single-filer thresholds, which can raise Medicare premiums even if income barely changes.

Selling in stages can also work against him. Single filers with MAGI above $200,000 may owe a 3.8% net investment income tax on some or all of their gains. The table below applies the 2026 brackets in each scenario.

Using fixed 2026 Medicare brackets, the two-year split is the most expensive, with $15,370.40 in combined surcharges and investment taxes. Selling everything at once totals $13,575.20, while spreading it over three years comes to $13,860. These amounts exclude regular capital gains taxes and assume his other income stays the same.

Bonds Do Not Erase the Tax Effect

Buying bonds with the proceeds does not remove the gain from the tax return. In addition, the bonds can keep adding to MAGI in later years. Even if the move feels tax-free, SSA adds tax-exempt interest from municipal bonds back into MAGI. Treasury interest also counts as taxable income. A bond portfolio designed to reduce risk can still generate enough income to place him near the first surcharge bracket. We have explained IRMAA and other premium traps retirees can encounter, and you can read our free Medicare guide here: Medicare’s Hidden Bills.

Form SSA-44 does not provide a blanket escape. Social Security may grant relief after certain life events, including retirement, reduced work hours, or the death of a spouse. A voluntary sale of a fund does not qualify on its own.

Three Checks Before He Places the Order

1. Rebalance inside retirement accounts first. If he owns stocks in an IRA, he can sell them there and buy bonds there, then leave the taxable fund alone. Heirs typically receive a new cost basis based on the investment’s value at death, which can wipe out decades of unrealized gains.
2. Identify each tax lot. Use specific-lot identification and sell the highest-basis shares first, beginning with the most recent reinvested dividends. Then compare the expected gain with the bracket edges at $137,000, $171,000, and $205,000.
3. Choose the tax year intentionally. A December sale sets the premium two years later. A January sale pushes the impact back by another year and gives more time to gather basis records. If he decides to split the sale, he should commit to the full plan before making the first trade.