The Collaborative International Dictionary of English v.0.48:

Exchangeability \Ex*change`a*bil"i*ty\, n.
   The quality or state of being exchangeable.
   [1913 Webster]

         The law ought not be contravened by an express article
         admitting the exchangeability of such persons.
                                                  --Washington.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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