The Collaborative International Dictionary of English v.0.48:

Interchangeability \In`ter*change`a*bil"i*ty\, n.
   The state or quality of being interchangeable;
   interchangeableness.
   [1913 Webster]

WordNet (r) 3.0 (2006):

interchangeability
    n 1: the quality of being capable of exchange or interchange
         [syn: exchangeability, interchangeability,
         interchangeableness, fungibility] [ant:
         unexchangeability]