From an old version of the rules. No mention of code share being exempted. Let us know UA continues to decline your re-faring.
United Airlines (UA)

Rule 1C
Except as otherwise provided within specific fare rules, transportation is subject to the rules, fares, and charges in effect on the date on which the ticket is issued. … The provisions of this rule apply only to the passenger to whom the ticket was originally issued. If, after a ticket has been issued and before any portion thereof has been used, either a decrease in the fare or charges applicable to the transportation shown on the ticket becomes effective, or a new fare (unless that new fare such as a certain limited sale time promotional fare is specifically exempted from this provision) for which the passenger can qualify is added between the points shown on the ticket, the full amount of the difference in fares will be refunded and any applicable service charges waived (except as specified in this rule) provided: There is no change in origin, destination, stopover point(s) / flight(s) / date(s) shown on the original ticket: Where the decreased fares or charges or the new fares bears a first date on which tickets may be issued, the originating flight is not earlier than the first date which would be allowed after meeting advance reservation and ticketing requirements applicable to the decreased fares or charges or to the new fare; Subsequent to the decrease in fares or charges or the addition of a new fare, all conditions of the decreased fare or charges or the new fare are met, including booking code and advance reservations and ticketing requirements. Exceptions: The passenger will be given the option of receiving a refund (after subtracting the USD $75 service charge), or Receiving the difference in fares in the form of a non-refundable miscellaneous charge order (MCO) or United Airlines travel certificate, The USD $75 service charge will be waived if the refund is issued in the form of a non-refundable MCO / UA travel certificate. The MCO / UA travel certificate is transferable to another person, valid for one year from date of issue and may be used as payment for air travel or travel-related services on United Airlines or United Express only. Partially used MSOs / UA travel certificates will have residual value.
Rule 1 C / 60th revised page UA-2 / November 11, 1997

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