It is easy if the website lets you change the ticket. I just spent over two hours yesterday changing a flight by two days. It was booked with companion cert. The website didn’t let me change the ticket and ask me to call in (happened many times when I use companion certs.) Original fare was $259, and new fare was $189. With $70 credit back, I just texted 82008. Waited 20 mins to get to someone. Then another 1 hour 45 mins for the ticket to be changed and fares recalculated manually. Then was told I had to call in to pay $0.16 in tax for the companion. Luckily when I called the 75K/100K line, somebody picked up right away with no wait. I explained I was asked to call in to pay $0.16, we both laughed, and then I was warned the wait could be long. Once again luckily, she was able to get in touch with accounting and charged my card on file fairly quickly. The whole process took around 2.5 hours. And I only got $47 in credit instead of the expected $70. Not worth the time so I didn’t pursue further.
Three weeks ago I called in to change a companion cert ticket from FLL-SEA to MIA-SEA because again the website didn’t allow me to. No wait to talk to someone, but it took over an hour for the ticket price to be recalculated manually. So be ready it could take some time.