I had booked DRO-DEN-ONT on UA with Lifemiles. After a flight cancellation, they automatically put me on DRO-DEN-SFO-ONT. Unfortunately, the Avianca site would not allow me to manage this booking online. I called AV and asked if they could change me to DRO-DEN-LAX, they could not. However, while working on my reservation, they cancelled my DRO-DEN-SFO-ONT reservations, and then said that they can not get it back due to no availability... As I've seen others post about, she instructed me to call UA and have them create space for me again on the DRO-DEN-SFO-ONT flights.

While on the call, I am seeing the DRO-DEN-LAX flight that I wanted is available on lifemiles.com for the same number of miles I redeemed for the original reservation, so I asked that they refund my booking so I can rebook on my own. After being transferred to a Lifemiles agent, they agreed to issue a refund of miles and taxes.

So, while I did ultimately get the flights I wanted, it was certainly in a roundabout way.