Versioning and the transition between versions can be one of the more challenging aspects of designing and operating an API. As such, it is best to start with some mechanisms in place to mitigate this from the start.
To prevent surprise, breaking changes to users, it is best to require a version be specified with all requests. Default versions should be avoided as they are very difficult, at best, to change in the future.
It is best to provide version specification in the headers, with other
metadata, using the Accept header with a custom content type, e.g.:
Accept: application/vnd.heroku+json; version=3