only two possibility to waive change fee on heavily discounted fare tickets
1. UA needs to change your flight (such as they cancel the 787 Denver to NRT
route)
2. One of the stopped cities expect weather alert on your travel date and UA
announces it in daily operation important notice (usually one day before
the trip). For example if travel from/to/through snowing ORD today or
tomorrow, you can change schedules free.