Rename a key within a map without modifying the rest of its contents.

_data.yaml

---
id: 1
type: book
meta:
  format: .mobi
  modified: false
---
id: 2
type: book
meta:
  format: .epub
  modified: true

rename_key.yaml

#@ load("@ytt:overlay", "overlay")

#@yaml/text-templated-strings
---
#@ def rename_key(orig, old_key, new_key):
#@overlay/remove
(@= old_key @): null
#@overlay/match missing_ok=True
(@= new_key @): #@ orig[old_key]
#@ end

#@overlay/match by=overlay.all, expects="1+"
---
#@overlay/replace via=lambda left, right: overlay.apply(left, rename_key(left, "format", "ebook"))
meta: