Caveat: I don't know United's practices quite as well as industry practices in general, but in general ...
There are two ways to change a ticket: reissue, and revalidation. If there is no change to origin, destination, or routing, then it is possible to make the change via revalidation rather than reissue. A new ticket is not issued in the case of revalidation.
Under normal circumstances, a change to the first segment would cause the entire trip to re-price, and you'd likely be out much more than the $300 change fee. It's pretty standard practice for airport agents to override that behavior in the customer's favor if the customer makes an honest mistake (e.g. the flat tire rule). It is possible for them to override that behavior while reissuing, but when it's practical to do so I imagine it's much easier for them to do it with a revalidation. I suspect that is the case here.
Originally Posted by
docbert
The relevant details are in the
Contract of Carriage :
RULE 7 TICKET VALIDITY PERIOD
[...]
NOTE: Nonrefundable fares have no value after ticketed departure time. EXCEPTION: When the Passenger cancels the ticketed flight reservations prior to the ticketed departure time,
the ticket will be valid for transportation for one year from the date of issuance of the original ticket and will be subject to any and all applicable change fees. Otherwise, the ticket has no value after ticketed departure time.
You didn't seem to read the rest of that section, which states:
Ticket issued by UA or its authorized agent on UA Ticket Stock will be valid for transportation for one year from the date on which transportation commences at the point of origin as designated on the original Ticket or, if no portion of the Ticket is used, one year from the date of issuance of the original or reissued Ticket, whichever is later. When an unused published fare Ticket is completely reissued, the new Ticket validity on the reissued Ticket will be determined from the date the Ticket was reissued.
OP was assuming an unused ticket was reissued, and that would extend validity. If the ticket was reissued, that would be the case (I have boatloads of personal experience with this), but it sounds like, for some reason or another, it was not reissued here.