“Yes and” doesn’t always mean literally saying yes. It’s about accepting the premise, not the proposal. The way it was taught to me, if you offer me a live hand grenade, I can accept the premise by saying “oh god he’s got a bomb!”, even though I’m saying no to the concrete offer to take your hand grenade.

“Let’s change everything” - depending on context - can be a no to the premise. It’s the same as the Pixar idea - improv is about making your scene partners look good by supporting their choices, and building on them in turn.