For existing apps normally you don't need to do anything. Your apps should keep on working, at least in the short term. Long term Apple is likely to bring in new devices and OS features that cause problems for older apps, but that normally takes years to happen.
It is only apps that are new or being updated that need to adopt such guidelines. So if you put out an update after June 30 and have not done so already you will need to adopt all of the practices in that guideline that are relevant.
Even if your app does not need updating, it might still be worth pushing out an update following these guidelines, as that will make your app more proof against future OS and device changes.