However if the statement "we can change this at any time" was contradicted by another statement in United's advertising saying "if you do X, you get Y" then I'm not convinced that the former overrules the latter.

I had a quick look at the current United site to see precisely what it says about benefits, but the current phrasing is extremely careful not to claim to give you anything at all: "Premier benefits in subsequent Premier program years may not be the same as those contained herein. See current Premier benefits for more information."

I have no recollection of whether the old United site said the same thing, however. If it didn't, or if some other piece of official literature failed to contain such ...-covering phrasing then he might have a case.

Look, I'm happy to sit back and let this dude have his class action. If he wins then it's a bonus (probably a miniscule one) for me, if he loses then it's no skin off my nose. It's not clear to me that he doesn't have a case (like I said I think he has a moral case but I'm not sure about the legal one).