At the risk of repeating the previous posters, a summary of the salient points:
* With a J class ticket you can select a seat online at anytime once ticketed
* The seats available for selection will be influenced by your One World frequent flyer status (emerald, sapphire, ruby); for example, if you are emerald all seats in the J cabin will be available that aren't already occupied. However, if sapphire or below rows 11-13 on an A380 are often restricted to emerald FF.
* Seats that are unavailable for selection due to FF status can become available closer to departure date