Why? This is pretty straightforward.

It's possible segments were booked on separate fares even with a connection less than 4 hours. The "Fare Details:" line in email receipt would confirm whether or not it was booked on a single fare.

didn’t work for me on the app, but not sure if that has to do with what’s going on now or just DL IT limitations in general (or what xliioper mentioned upthread).

If I have a revenue ticket purchased in C+ and the flight I want to SDC to only has seats in Y, does my fare class get changed to Y? Or, is there another fare class for “originally booked in C+ and downgraded to Y”? And if the fare class gets changed to Y, do I still get upgrade priority over pax who booked in Y?

There’s too many details you haven’t shared. Route? One ticket or two? RUC/GUC? Companion pass?

SNA-SLC-JFK.
One, one-way award ticket.
No upgrades or companion passes applied.

Maybe someone else can share their experiences but IME the app should have been able to handle a SDC with what you’re describing.

IME this "should" be easily handled by the app but the app often barfs on "easy" itineraries that it "should" be able to handle.

if there have been any IROPs it's nearly useless, if there have even been schedule changes it seems to screw up more.

also just in general it seems like it worked pretty well when this feature was new and has progressively gotten worse and worse over time

IME, your fare class gets changed to Y, but you'll have priority on the upgrade list over pax who book in economy.

I can never remember this, but if I purchase main cabin and use a RUC to confirm an upgrade to FC, does the SDC last seat available in F apply?
I got pushback from the check in counter agent at PHX, but the agent did move me in to the last F seat (after telling me the policy was I would be put in coach and on the gate upgrade list with waitlisted RUC).
Was the agent correct, or was I?

Its supposed to be normal SDC from F - meaning last seat. However, many agents don't know or understand how to do this correctly.

If your RUC is confirmed AND you reticket, then YES, by the book you're entitled to the same SDC rules as paid F. There is a note in the DL knowledge management system about this, see this post:

https://www.flyertalk.com/forum/delt...l#post33651818

thanks to you both, yes my RUC was confirmed and re-ticketed. And thanks for that link.

Still having issues trying to do SDC.........upcoming trip tomorrow (award ticket booked in OZ)

1st Attempt: Online and in the App (gave error)
2nd Attempt :Diamond Line phone agent (wanted an additional fare collection of 30,000 miles or I could standby for this flight)........attempted to quote the rules and even told her sometimes "it has to be done manually" and she pushed back......ended the call and gave her a 1
3rd Attempt: Diamond Line phone agent (spouted off some word diarrhea about why she couldn't do it.....once again attempted to quote the rules and she pushed back.....ended the call and gave her a 1
4th Attempt :Via Chat......agent took care of it in just a few minute with no pushback

While I am not a big fan of using the chat feature and echo many of the comments on here I have had almost PERFECT success doing SDC via chat, where my SDC via phone is typically 30% or so. What I am learning with the phone agents is if the "easy" button doesn't allow them to do it then it can't be done. DL is training their phone agents to put total confidence in there top of the line IT department, which we all know is not the case.

If I'm flying AAA-BBB-CCC on paid F, is it possible to SDC to a later BBB-CCC once I arrive at BBB? The layover at BBB would be longer than 4 hours, but same-day.