Change an API without breaking callers

I need to change this endpoint: <paste current shape and the change>
It's used by: <my own frontend only / other people>.

Tell me first: is this actually breaking? Adding an optional field
usually isn't; removing one or changing a type is.

If it's breaking:
- How do I ship it without breaking existing callers — a new version,
  a new field alongside the old, or a transition period?
- How long do I keep the old behaviour, and how do I find out who's
  still using it?
- What do I tell callers, and when?

If it isn't breaking, say so and let me just ship it.