Thank you, I guess, for this thread. It turns out that my ticket number changed, probably because the departure time changed.

But did I need to know this? The airline (Delta, so maybe this is the wrong forum) invited me to check-in online, which I intend to do. If I hadn't known the ticket number had changed, what difference would it have made? Just to be clear, it is the 13 digit numeric ticket number that changed and not the six character alphanumeric identifier that I believe is the PNR.

Thank you for the continuing education that I get from FlyerTalk.