No one loses coverage as a result of converting to MAGI rules, but, in states that don't adopt the new adult eligibility group, it is possible that some individuals will lose coverage.
The Affordable Care Act ensured that no one would lose health coverage--if they were not eligible under the new MAGI standards either they would be covered under the new Medicaid adult coverage group or they would be able to purchase insurance through the Marketplace with the benefit of a premium tax credit and likely cost sharing reductions. Following the Supreme Court's decision, the Medicaid expansion is voluntary for states, and in states that do not adopt the new coverage group some individuals may lose coverage at the time of their renewal when the new rules are applied.