Set the 4 new Expedition cards aside at the start of the game. When you play a Modifier or Untimely Death with an Expedition ring at lower right, resolve the card's immediate effects first, then place the Expedition of that color in the center of the table.
Only 1 Expedition can be in play at a time, so it replaces any previous Expedition out already. The new Expedition adds a rule that affects all players as long as it's in play.
Some cards also have special effects that trigger if a particular Expedition is already in play.