Update:
I decided to try for a 5th attempt and this time I started the call with the DM agent by politely explaining I'd spoken with a few other DL reps and believe I was told inaccurate info, so was hoping she could please look into it. After explaining what I was looking to do she initially came back with the exact same response as the previous 4 agents. However, this time I told her that because I cleared the GUC earlier this week the way it should be looked at is the same way as if I had a premium award ticket and wanted to change flights - there didn't need to be premium award inventory to do a SDC, just a premium seat available. At that point something clicked with her and she said "let me see if this is something I need to manually re-issue," and she put me on hold.
Four to five minutes later she came back and said she was still working on it. Another minute or so later she came back and said you're all set and confirmed. So in the end it finally worked, but really wish it didn't take 5 calls, 3 of which included the Help Desk.
DELTA - if you're reading this post - please, please, please train your agents better. The fact the one's working the DM line aren't informed about your benefits to medallions is scary for your business. These are your elite customers, treat them as such. Get them educated agents at a minimum.