I have implemented Fcm in my App and thats working great.. In My app i am creating notification groups by
$data = array();
$data["operation"] = "create";
$data["notification_key_name"] = "gcm";
$data["registration_ids"] = array("token");
and iam successfully received my notificationkey for frequent use... and if the token changes to another should i remove the tokenid from the notification group and add the new one everytime ?? or fcm will take care of it???