Bidkat, when using miles or certs attempting to upgrade at the airport off of the Departure Management List, time placed on the list is very important (at least it is at United).

I've never found a "legal" work-around at United (who has a 4 hour check-in rule on your first flight of the day) to defeat the advantage that connecting passengers have being placed on the DM list at their originating airports, much, much earlier.

I can only rationalize my disadvantage sometimes, in knowing that this rule also works to my advantage, sometimes.

You may know or find an angel or supervisor at a particular airport that might have the authority or willingness you check you in and DM you early. But as a matter of rule at many airlines, you are just out of luck until legal check-in time.

Good Luck!