My significant other and I have a deal. If this happens to us, the first time, he will take the upgrade. The next time it happens, I will take the upgrade: and then we will keep alternating. The one exception being if it's a work trip and I have to work the next day.

On a Virgin Australia this once happened and I have away my upgrade to him. The flight attendants were very surprised and gave me special treatment back in economy, haha.