i booked HNL-GRU in D1 early January using GUC’s. I’m not sure this is solely a GUC issue since it pertains to a Domestic leg on the return do started a new thread. I think this may be a broader policy issue....
When the agent helped me with the booking the return leg from GRU-ATL-HNL was the easy leg, confirmed immediately in D1. The outbound leg was HNL-SEA-JFK-GRU and the SEA-JFK was not confirmed in 1st. That was okay so I booked.

On 4/4 Delta sends an email confirming seat assignments, on DL837 (ATL-HNL) I’m 5A. On 4/11 they advise of a schedule change (2hr earlier departure, still have 3 1/2 hrs between GRU arrival and HNL flight) and again confirm 5A seat assignment on the D1 flight.
on 4/14 they advise me of a schedule change, 837 crossed out and a replacement (2 min earlier) with new flights in D1 to LAX and then a First seat in a 757 to HNL. Clearly an inferior itinerary, so I then checked online. DL837 was still scheduled at the same time and had 25 seats in D1 available. BTW,The fares were not even that high....so what gives? What happened to my “Confirmed “ 5A seat now showing available on the seat map?

I just got off the Diamond line and after 25 min, including a Supervisor “check”, I was told “too bad”. Yes there were plenty empty seats but none available for a GUC at this time. I asked what happened to my confirmed seat, she said she couldn’t answer that question. I could waitlist that DL837 D1 flight or keep the confirmed Firsts , that was my choice.
I then asked the agent if she had dealt with Delta taking a confirmed seat away from a Diamond before, and shuffling them to inferior flights (longer duration, poorer seat configuration and service). She awkwardly said Yes, but by that point everything was awkward. This has never happened to me before and I’ve been Diamond since inception and used GUC’s throughout. They’re admittedly getting harder to use, but once paid and confirmed with a seat assignment I assumed we had a “contract”.

What gives with Delta’s new policy of after (multiple times) having confirmed a seat let’s go ahead and screw the loyal Medallion member trying to use the GUC’s Delta offers as its premier benefit?

Was your ticket reissued in D1 or was the GUC upgrade just confirmed? That seemingly can make a difference and I suspect that with your FC waitlist on the domestic segment in the other direction, DL has been waiting rather than reissuing into D1 for only the one direction (or reissuing both directions and arguably not permitting the GUC in that direction to potentially cover the segment where you're still booked in coach).

You can find whether a new ticket was issued (although not why it was reissued) by comparing ticket numbers or looking for a new ticket receipt (with ticket number--not just the PNR or itinerary email)in your email. The delta.dumb website should show you the current receipt (with ticket number), IIRC as a PDF while you should have a receipt from the time the ticket was purchased that shows the original ticket number. IIRC there was a way to click on the ticket number and see the whole history of previous ticket numbers ("exchanged for"), but that functionality might be gone.

The thread title is a bit confusing in that some DL 757 aircraft have a D1 rather than FC cabin, with real D1 seats in a 2-2 configuration but set at a slight angle and slightly staggered to facilitate aisle access for passengers in window seats. IIRC these birds have 16 FC seats.

NO, that makes it too easy for the recipient to just respond with something like "you're right, I can't help."

It's more effective to take a more positive attitude that while you're asking for a favor you expect it to be granted/possible rather than being negative about your expectations when you ask.

Of course they can see the upgrade, but IME once the ticket has been reissued DL views it as a premium cabin ticket, while if it hasn't been reissued yet YMMV.