Under the Affordable Care Act (ACA), changes in income and family circumstances are likely to produce frequent transitions in eligibility for Medicaid and health insurance Marketplace coverage for low- and middle-income adults. We provide state-by-state estimates of potential eligibility changes ("churning") if all states expanded Medicaid under health reform, and we identify predictors of rates of churning within states. Combining longitudinal survey data with state-specific weighting and small-area estimation techniques, we found that eligibility changes occurred frequently in all fifty states. Higher-income states and states that had more generous Medicaid eligibility criteria for nonelderly adults before the ACA experienced more churning, although the differences were small. Even in states with the least churning, we estimated that more than 40 percent of adults likely to enroll in Medicaid or subsidized Marketplace coverage would experience a change in eligibility within twelve months. Policy options for states to reduce the frequency and impact of coverage changes include adopting twelve-month continuous eligibility for adults in Medicaid, creating a Basic Health Program, using Medicaid funds to subsidize Marketplace coverage for low-income adults, and encouraging the same health insurers to offer plans in Medicaid and the Marketplaces.