Short answer: The old self-funded plan does. The new insurer covers services received after its effective date, so the employer must fund and administer run-out claims, and stop-loss covers them only if their paid dates fall within the policy’s window or a terminal liability extension.
Example: a claim is incurred December 20, 2026 and paid August 15, 2027. Under a 12/18 contract beginning January 1, 2026, the paid window ended June 30, 2027, so the claim is not reimbursed; under a 12/24 contract it would fall within the window if it meets every other condition. A fully insured policy starting January 1, 2027 does not pick up the December 2026 service.
Before terminating, inventory pending claims, large cases, appeals, recoveries and reserves, and confirm in writing who will adjudicate and pay old claims, submit stop-loss requests, keep records and answer employees, and how long the employer can still access its claims data.
Late items such as corrected provider bills or Medicare recovery demands can arrive long after the plan year, so keep funding and TPA arrangements for the old plan in place until they are resolved.