Customer service has already assembled a set of answers for obvious questions. Any players with more than eight characters in total will be locked out of playing until some of those characters are deleted, so if you've got characters on several PvE or PvP servers, you might want to delete some. Players can also expect banks to be merged (with an overflow tab if you need to withdraw some items) and all vanarch statuses to be reset, with refunds to active candidates. The full producer's letter has some other things for players to look forward to during September and beyond.
[Thanks to Marisa for the tip!]