tontybear wrote:There is nothing to stop you asking at check-in.
Often the usual rules are conveniently ignored re fare codes etc
However, they're more likely to be ignored on a full flight than one which is as wide open as the OP's.
Twice I've upgraded on an non-upgradable fare at the airport. The first was in LAX when they were looking for volunteers with non-connecting flights which excluded us. However it was well before the days of V-Flyer and out knowledge of upgradable fare buckets. We had simply decided to try and upgrade before we even went to the airport. We were among the first to check-in for that flight and asked if it were possible. We were asked if it was with miles or cash and having said miles, with a few strokes of the keyboard, we were through.
The second time was in IAD. I was in PE on a K fare but a Gold member, and having arrived in the CH there, on a whim asked if an upgrade was possible. Within minutes I was sorted.
The moral of the story is to ask. The worst they can say is no but that will depend on the loads.