Very interesting SDC experience the past weekend.
Seg 1, original LAX-SJC, changed to LAX-SFO
Seg 2, original SJC-SEA-FAI, changed to earlier SFO-SEA-FAI
Seg 3, orignal FAI-ANC-LAX, changed to later FAI-ANC-SEA-ONT, changed again to FAI-ANC-PDX-ONT

First change was easy, agent need to go back and confirm because it's a co-terminal change.
Second change was a little difficult, because SFO-SEA I want is sold out in main, but 2 F open. I don't know what magic the agent did, I was waitlist in main first, very quickly moved to PC, then to FC in less 1 hour.
Third was very hard.
Attempt 1, calling reservations.
1. It's a new agent, don't even know SDC can do co-terminal change.
2. I am adding one more segment.
3. My original flights all departs before midnight, now it is spreading into 2 calendar days.
After 20+ minutes, another agent took over without me asking. I believe the second one is a much more experienced one. After another 20+ minutes, finally done, with collection additional $4+ extra in fees.
Attempt 2, move up the FAI-ANC shuttle.
The check in agent was even newer, all she can is to move my FAI-ANC up. And even with that, she went back and after 10 minutes, came back with my newer boarding bass apologizing she was new there.
Attempt 3, final change to the routing.
There was no CS agent in ANC at the very busy evening departure rush hour! My calls to reservations keep dropping, and each call back is longer until it's over 1 hour even for Gold. I caught a GA finishing up boarding a flight, and asked him to help out. I told him my calls were keeping dropping, at one point, I was so close to finish things up, but the phone agent took forever to even load a seat map. He called using the phone at the gate to some special number I assume, route changed after about 5-10 minutes on the line with them. However, false promise I was on the upgrade list though.

Eventually, I did see glitches in the app for sure, like I had two FAI-ANC back to back at the same time etc, but I was happy I got on the itinerary I like, otherwise I'll need to pay a lot $$$ even at the time of booking.