Plan an agent's part in porting untested payment code

Your company is retiring an old payments gateway. A 4,000-line Java module talks to it today: it builds charges, handles partial captures, retries and refunds, and rounds amounts in several places. It has no tests, and the people who wrote it left years ago. It must move to the new internal payments API this quarter, and you want an AI agent to do most of the work.

How do you run this? Decide what the agent does, what you keep, and where you check before real money flows through the new path.