Replacing center cards in Ascension
I have only played one game of Ascension, and I got the same idea.

Here is the ruling directly from the download-able FAQ:


  Q:  When does a card in the center row get replaced?  If I defeat Mephit, do I banish a card 
  in the center row before or after a new card replaces the Mephit?
  
  A:  Whenever a card leaves the center row, the first thing that happens is that it is replaced from the center deck.  There should be six cards in the center row at all times.  In this instance, if you 
  defeat Mephit, you replace the Mephit from the center deck and then make your decision about 
  what card to banish. 


FAQ Link