Any old listings just show what this one does - "partial refund available within 30 days." It is annoying that listings I made awhile ago still show as accepting returns. Once I change my return policy, all my listings should reflect the current return policy. I don't know how to change these old listings.I have no idea of the answer to your question. Are you able to look at an old listing that way sold long before you changed the policy?
I wonder if you contact them at support, if they can see your old policy.
ETA: I'm trying the fancy bulk editing tool but there are 13 jobs ahead of me in the queue so I don't know if it will work.