The grand finale includes a final save maintenance for characters on November 1st, 2012. World servers will then be shut down on November 11th at 3:00 a.m. EST. At this point, the plan is to keep the game off-line until launch of the alpha. However, Square Enix is offering the chance for players to vote on whether or not to bring the servers back up in order to play together during the wait. The caveat is that all progress and changes will not be saved and carried over to the new version.
The letter also discloses information about the upcoming alpha test. The test will be in four phases; the first will be restricted to Japanese residents only. Beta will not follow until the producer is personally 100% happy things are ready for that stage.
For further details, check out the full producer's letter.