The Collaborative International Dictionary of English v.0.48:

Replaceability \Re*place`a*bil"i*ty\ (-?-b?l"?-t?), n.
   The quality, state, or degree of being replaceable.
   [1913 Webster]

WordNet (r) 3.0 (2006):

replaceability
    n 1: exchangeability by virtue of being replaceable [syn:
         replaceability, substitutability, commutability]