Versions are dates. Your account is pinned to the version current when you first called, and stays there until you deliberately move.
Commerce Census-Version: 2026-09-01
Removing or renaming a field. Changing a field's type. Changing the meaning of an existing value. Adding a new required parameter. Tightening validation. These require a new version.
Adding a new field to a response. Adding a new optional parameter. Adding a new endpoint, event type or enum value. Write clients that ignore unknown fields and tolerate unknown enum values, and these will never break you.
Pin the new version per-request first to test it in isolation, then move the account default. Old versions are supported for at least twelve months after a successor ships.