The newest scan for the upcoming Final Fantasy IV Advance has answered some questions about the additions to the game. Specifically, it outlines how the character switching will take place and some details surrounding the new dungeon.
Character switching will take place in Mysidia in the tower. All the available characters will be gathered there, and by talking to one of them, they can trade places with a member of your current party. It seems this function will only be available for characters that can make it to the tower, as not all characters are available.
The new dungeon will be 50 floors in length, and its purpose will be to level the other characters and give them ultimate weapons similar to those found in the original game's final dungeon. Once again, a special boss monster will guard each one, and they will be unique to the GBA version. The scan shows Porom getting a "Seraphim Mace" as her ultimate weapon.