Slon747Можно ли проверить, как станет вести себя мое приложение с 28.06.2017?
Там же и написано: Familiarize yourself with the new v2 syntax. Read up on the patterns of v2 endpoints, do a tutorial for an SDK, and test-drive v2 calls with your own data in the API Explorer .

Ну и какое-то время обещают поддерживать гибридный режим: Hybrid mode

You can use both API v1 and v2 at the same time in an app. This works for the HTTP API, the Java SDK, and the Python SDK.
To do this with the HTTP API, you can just make regular API v1 calls with the appropriate URL structure and method. The SDKs mentioned above have the v1 functionality built-in. For both Java and Python, just follow the v1 tutorials to create a client and use it to make v1 calls.

То есть, скорее всего, не одномоментно 1-ый отключат.