I assume it's just clarified wording - permitted any time provided there's still a seat left in that cabin (i.e. not "any time, all the time, any situation") OR for upgrade to higher cabin (hence the fare difference bit). And refunds would be in full for unused tickets or calculated based on flown sectors for part-used tickets. No doubt some artards thought they could get a full refund after flying one leg and protested the site wasn't clear enough or something?

Or maybe I'm just in too charitable a mood today and am giving BA the benefit of the doubt for once!