Facebook announced a pending automatic upgrade of Graph API users from v2.3 to v2.4. Facebook will implement the upgrade on July 10, 2017. Accordingly, developers using v2.3 need to migrate API calls prior to July 10th. The most current version of the Graph API is v2.9.
Facebook's upgrade comes in the ordinary course of its two-year deprecation cycle. v2.4 includes a significant change to improve performance (i.e. a reduced field set). In addition to the reduced set of fields, some v2.3 features will be deprecated. So, it is critical that developers migrate to at least v2.4 prior to July 10th, or apps will be affected.
When migrating to v2.4, developers can choose the fields desired for return with the fields parameter. To assist developers, Facebook has published an API Upgrade Tool. The tool helps developers understand calls that will be affected by the upgrade. Developers can also test their apps at the developer site.
While the automatic upgrade will transition v2.3 users to v2.4, Facebook recommends that developers migrate to the most current version (v2.9). Upgrading to v2.9 will eliminate the need to migrate again in a few months. To get up to speed on the two year deprecation cycle, check out the changelog. Further, learn more about Facebook's versioning strategy here.