“United Express” isn’t an airline. There is absolutely no question that a change in operating airline is grounds for a full refund. It’s possible, I guess, that UA has decided that they’d rather call people’s bluffs and force them to take a refund rather than provide a free change, but I don’t really see the upside for them in doing so. I suspect HUCA will be successful — just tell them you’ve had bad experiences with Commutair. I suspect you wouldn’t even be lying.