One file per change to what a design file saves that no release has shipped yet. Adding a file here is the whole format bump: never edit FORMAT, JSON_STEPS or another change's file.

Name it after the change, lowercase with dashes: placed-plane-origins.rs.
Start it with a //! note: what is saved, what an older design gets without it, and what the last release does with a design that has it (it refuses the newer format number).

If older designs must be rewritten as they open, add the step to the same file:

//! …the note…
use serde_json::Value;

pub fn step(v: &mut Value) -> crate::Result<()> {
    // rewrite the design's JSON here
    Ok(())
}

Steps run in file-name order, after the steps of every older format.

While this folder has a change in it, the build writes the next format number (the last folded one + 1, which no release reads), and lists the changes a file has in its format_pending field, so a later build applies only the steps that file is missing. At release time scripts/fold-format-pending.sh moves these files into ../v<N>/, which fixes that number for good. See crates/doc/src/format.rs and CONTRIBUTING.md.