Just a quick update for anyone who cares!
Called the diamond line and was first told there isn't anything they can do to help since I was transiting on to ATL (!). HUCA. Agent says there is a 10:00 AM but it's completely sold out, I ask her to put me on standby and she says she can't do that without having a confirmed Delta ticket, I reason with her that this is still a 006 ticket (even the UA receipt read as such) and asked her to check with her supervisor. She comes back on the line 8-10 minutes later saying a seat opened up and they're able to confirm me on the 10:00 AM.