A few weeks ago I purchased a OW fully refundable ticket direct from Delta. I later made a flight change in the app, and the flight I selected was marked as an even swap refundable ticket (+$0 difference). I then needed to make another flight change, this time it wouldn't let me in the app/online, so I contacted an agent who was able to change my flight and indicated there would be a refund as the new ticket was cheaper than the one I originally purchased.
A few questions about this.
1) I never received a refund email from Delta, I only have the chat message from the agent. Will this difference be refunded?
The flight receipt email shows the original payment amount (shown as an ecredit in the receipt) being applied, and that the amount of the new ticket was less than that ecredit amount. Why is this being shown as an ecredit? For reference when I search this ecredit, I get this message, "We’re sorry, but a portion of this eCredit has been used and fare rules prevent the redemption of any residual value."
2) Is this ticket still refundable in the future? The flight receipt email has the T&Cs indicating the ticket is non-refundable unless the original ticket was refundable, so I assume it's refundable. However if I go to change the flight (as this is one way to check as noted here on FT), the options shown are all non-refundable.

ETA: I checked the fare rules code from my receipt, and it's HAUOA0MC. According to https://pro.delta.com/content/agency...structure.html, the 4th position indicates refundability, and 'O' is not refundable. My previous 2 fare basis had a 'Z' in that 4th position, which is refundable according to that page.
Why would the agent change the flight but it then results in the new ticket being non-refundable? Is that normal process or did this agent make a mistake?