From the fare basis code, I am most certain that this is an unrestricted ticket, implying that when making a change, the original ticket will simply be exchanged for a new one. There are no penalties. If the new fare is lower, the difference will be refunded. If the new fare is higher, you pay the difference only if you still intend to use the new ticket. For OP's case, having already arrived at the new destination, UA likely won't pursue the add-collect that results from a higher fare.