Well, the changing to higher cabin allows you to waive the change fee even if you change flights, destination, days, market, etc. Anything. I believe that's in all change fee verbiage for ACs public fares (like the real ones not the nicely formatted website ones).

For anything else, as long as you're on exact same flights, on the exact same day, at the exact same time, you're OK. But you can't move from 1600 to 1500 YULYYZ, upsell tango to flex, and then get the change fee waived.

But things change all the time so 🤷.♂️