Retirees with large 401(k) balances face a crucial five-year window between retirement and Medicare enrollment to convert funds to a Roth IRA strategically. Conversions before age 63 avoid higher Medicare premiums tied to income reported two years prior. Converting about $200,000 annually from ages 60-62 at a lower tax rate can reduce future required minimum distributions and Medicare surcharges. Paying conversion taxes from taxable accounts and delaying Social Security benefits to age 70 optimize this strategy. Ignoring this window leads to costly Medicare surcharges for years.