The Collaborative International Dictionary of English v.0.48:

Mutability \Mu`ta*bil"i*ty\, n. [L. mutabilitas: cf. F.
   mutabilit['e].]
   The quality of being mutable, or subject to change or
   alteration, either in form, state, or essential character;
   susceptibility of change; changeableness; inconstancy;
   variation.
   [1913 Webster]

         Plato confessed that the heavens and the frame of the
         world are corporeal, and therefore subject to
         mutability.                              --Stillingfleet.
   [1913 Webster]

WordNet (r) 3.0 (2006):

mutability
    n 1: the quality of being capable of mutation [syn:
         mutability, mutableness] [ant: fixity,
         immutability, immutableness]

Moby Thesaurus II by Grady Ward, 1.0:

72 Moby Thesaurus words for "mutability":
   adaptability, adjustability, alterability, caducity,
   capriciousness, changeability, changeableness, changefulness,
   choppiness, corruptibility, dappleness, death, deviation,
   difference, differentiation, disorder, divarication, divergence,
   diversification, ephemerality, ephemeralness, evanescence,
   finitude, fleetingness, flexibility, fluidity, fugacity,
   impermanence, impermanency, inconsistency, inconstancy, inequality,
   instability, irregularity, jerkiness, malleability, mercuriality,
   mobility, modifiability, momentariness, mortality, motleyness,
   movability, nonconformism, nonconformity, nonstandardization,
   nonuniformity, perishability, permutability, plasticity, pluralism,
   raggedness, resilience, rubberiness, suppleness, transience,
   transiency, transientness, transitoriness, unconformism,
   unconformity, unevenness, unorthodoxy, unsteadiness, variability,
   variation, variegation, variety, variousness, versatility,
   volatility, wavering