You will be able to opt-into Replies through the Page admin panel in the Manage Permissions section where you will see a prompt to turn Replies on. Once you opt-in, you will be able to opt-out but all Pages will have the feature enabled on July 10, 2013. When you opt-in, you will see the feature show up on new posts on the Page. Also, the feature will be automatically turned on for profiles with more than 10,000 followers. At launch, this feature will only be available on desktop and we plan to make it available in the Graph API and mobile in the future.
After months of testing, we have seen how the new feature can improve conversations and be used to start open dialogues with the community.