Process a loan top-up — a MERL (Mid-term Early Renewal Loan): the borrower clears their current loan partway through the term and immediately takes a fresh loan, and the insurance continues on the SAME policy. In one all-or-nothing transaction the active policy is resolved from the original loan reference, the current cover period is closed (superseded), a new cover period is opened for the new loan (family members carried forward unchanged), the policy end date is extended to the new loan end date, and the premium payment is booked. The new cover is priced by the product's normal pricing model with the product's configured top-up premium factor applied (default 50%).

The response body is the continued policy in the exact representation the policy-creation endpoint returns, so one parser serves both journeys. Requests are idempotent on the top-up loan reference: replaying an already-processed top-up returns the identical policy body again, with the informational response header X-Idempotent-Replay: true — nothing is created or charged twice.

Rejections return HTTP 400 with an ErrorResponse whose message carries one of these error keys: