I had luck last summer when Hetfield got Covid and the second night of Metallica was moved to a week later, by which time I was supposed to have flown home. Most of these tickets revert to historical pricing once you’ve started the trip. I was able to find I class on new dates that suited, was also able to change gateway from LAX to PHX. Lost the upgrade to first as there was no suitable flights that had both I and A availability. Managed all this for free. Not even a change fee.

Similar situation for a colleague earlier this year although one leg had only R class so there was a small up charge.