Originally Posted by jsloan

You've quoted the voluntary changes section, which covers the mechanics of the change but doesn't actually include the charges, if any. Those would be in the 'penalties' section. For a B fare, I'd expect there to be no change fee -- a B fare is generally quite expensive. (For a couple of sample dates I put into the calendar, I got $1400 on a half-round-trip basis for the MEX-FRA leg in a B fare, compared to $443 for a V fare if it had been available).

In a quick survey, I don't see any B fares that start with FARE BY RULE for either UA or LH. FARE BY RULE would be pretty unusual for a B fare -- that's text that appears for fares that are calculated from a base fare using a rule (e.g., "start with the regular Q fare, apply the basic economy restrictions, and drop the price by $35."). B fares normally stand alone.

If you can provide travel dates, we should be able to look the (current) fares up. FWIW, that same quick look at the fares for MEX-FRA tells me that if you have UA flight numbers, you're buying a UA fare, but if you have LH flight numbers, you're buying an LH fare, regardless of who's operating the flight. That's not always the case, but it appears to be true on that particular route.