Essentially there will be two access levels when the transition occurs in February. New players (or Starters, in Gameforge lingo) get free access to the entire game. Limitations on free play include "reduced access to functions such as the chat and the auction house," though Gameforge has not specified exactly what that means as of yet.
Aion veterans will also enjoy free access to the complete game as well as reduced instance cool-down times and increased PvP coin rewards. There is a cash shop, of course, which will include avatar customization items that may be used to individualize your character. Gameforge's press release also mentions a Gold Pack option that will grant time-limited bonuses, and we'll have more details for you as they become available. In the meantime, check out the official Aion free-to-play website for more on the game's move to F2P.
[Source: Gameforge press release]