In Small World, how does the Spirits' ability apply to already declined races?
The official rules (PDF) say (emphasis mine):


  When the Race tokens associated with your
  Spirit Special Power go into Decline, they
  never count toward the limit stated in
  Entering In Decline (p. 6) regarding having a
  single In Decline race on the map at any
  given time. You may thus end up with two different races In
  Decline on the map at the same time and score for them both.


This means that when your active Spirits go into decline, they do not push out an existing declined race. If in the future you decline another race, the old race will then be removed, and so on.