Paying up to a higher fare (from a fare that has a change fee associated with it) does NOT entitle you to a waived change fee if you are changing your itinerary (flight dates, departure times, and/or routings). If Res doesn't charge you a change fee for a changed itinerary, you are benefitting from a mistake on behalf of the Res Agent, not the fare rules.

Many int'l fares do not have a change fee associated with paying up to a higher fare class, but only on the same flight. Again, this only applies to customers who keep their itinerary in tact (otherwise we would all just use this as a way to avoid the $150 change fee).

Even if you pay up to a changeable B fare, you will still get hit with the $150 change penalty (or whatever your fare rules state) in addition to the fare difference if you are trying to change your itinerary. Same thing if you are trying to buy up to a refundable fare (the original amount remains non-refundable, and if a change fee applied to your old fare, you must pay that before the ticket becomes changeable).

Correct me if I'm wrong, but I believe the intention behind this system is to allow Mileage Plus customers travelling on int'l segments in D, Z, Q, V, W, S, T to pay up to a fare that is upgradeable with miles (C, Y, B, M, H) on the same itinerary without paying a change fee as well.

Now, can anyone explain to me why domestic itineraries don't allow for this? Too many times I've seen customers who want to pay up to a higher fare class (for example, from S or T to "V" fares to qualify for The Great Offer), but decide against it because UA also wants the change fee. Any ideas why domestic fare rules don't make for the same allowances? Or am I mistaken about the intention behind allowing this on int'l itineraries...