On a recent MCI-ATL-FLL flight, the ATL-FLL portion was canceled before I boarded at MCI. When I attempted to board in Kansas City, I was told my boarding pass was not valid and I was no longer booked on the flight. It seems that Delta computers rebooked my whole trip automatically (not just the connection), but I had failed to check in on the newly booked reservation for the same MCI-ATL flight on which I obtained a boarding pass 45 minutes earlier.

So even though only my connection changed, I lost upgrades on both flights and nearly lost my seat altogether on the outbound.

On another similar trip, my outbound was delayed making it iffy as to whether I would make my connection. I did make my connection easily, but was informed that I had already been rebooked for the next day. A Diamond standing next to me didn't realize he had lost his seat until he saw it had just been given away to someone on the Upgrade List. We both had to standby for coach seats on our original flight.

I like that Delta tries to get me to my destination, but I don't understand why Delta's computer system insists on canceling reservations before i have had a chance to confirm that the change made is the one I prefer.