Originally Posted by aaronmk0

The Delta site *will* actually successfully change your seat, even though it will not provide a confirmation of the change. You will instead find that it has given you the right seat upon checking in.

(For me, this was only a problem on the Alaska Airlines codeshare section of my flight, which makes me think it's a problem with Alaska instead. That section also did not allow online check-in *or* check-in at a Delta kiosk, but I was able to check in at an Alaska Airlines kiosk.)