As far as I know there's no set rule for this. It's IRROPS and you're at the mercy of airline, (to a degree).

Just thinking from a layman's perspective: If they made this an official rule, it'd get very busy with the earlier flight and lot of disappointed passengers (only so much room on the plane). In effect, only the elites would get to enjoy the earlier flight anyway.

Seems like a case where it's better to try and catch the fly with honey and just politely ask, instead of insisting on rules, and/or use the airport version of HUACA: Head for the nearest lounge/reticketing desk and try with the agent there, instead of bothering with an obstinate GA.