I purchased a refundable domestic ticket several months ago (one seat in Economy Comfort (W) and one in First (A)), and I am now certain that I will be taking the trip so I would like to convert it to the current considerably cheaper non-refundable fare.

I would like to keep my current seats and flights.

I am thinking I should either
- call Delta and ask them to convert the ticket for a partial refund
- cancel the ticket and buy a "new" fare online without speaking to anyone at Delta

Does anyone have experience to share or recommendations for doing this?