Versioning your REST API is analogous to the versioning of any other API. Minor changes can be done in place, major changes might require a whole new API. The easiest for you is to start from scratch every time, which is when putting the version in the URL makes most sense. If you want to make life easier for the client you try to maintain backwards compatibility, which you can do with deprecation (permanent redirect), resources in several versions etc. This is more fiddly and requires more effort. But it's also what REST encourages in "Cool URIs don't change".
In the end it's just like any other API design. Weigh effort against client convenience. Consider adopting semantic versioning for your API, which makes it clear for your clients how backwards compatible your new version is.
