I was upgraded to Delta One twice on a HND-LAX flight last year, and both times I had stayed in the Main Cabin rather than take the Delta Comfort upgrade. I would be notified of the upgrade when going through the gate, and they would give me a slip of paper with my new seat printed on it. The Delta App showed that my seat had changed.

Every time I've taken the Comfort class upgrade on the same flight, I have not been upgraded to Delta One, even when it looked like it was available. Could it be that the chances of getting upgraded to Delta One is higher if you choose to stay in the main cabin? I am Platinum so I can select a Comfort seat at the time of booking, but if that ruins any chances of getting bumped up to Delta One, I have to weigh out the risk and benefits.

Any similar experiences to this? Is there a rule they go by for that? Automatic Delta One upgrades are not available for International flights as far as I know.