The Collaborative International Dictionary of English v.0.48:

Unchangeable \Unchangeable\
   See changeable.

WordNet (r) 3.0 (2006):

unchangeable
    adj 1: not changeable or subject to change; "a fixed and
           unchangeable part of the germ plasm"-Ashley Montagu; "the
           unchangeable seasons"; "one of the unchangeable facts of
           life" [ant: changeable, changeful]

Moby Thesaurus II by Grady Ward, 1.0:

125 Moby Thesaurus words for "unchangeable":
   abiding, accordant, adamant, adamantine, alike, automatic,
   balanced, cast-iron, changeless, consistent, consonant, constant,
   continuing, continuous, correspondent, dour, durable, enduring,
   equable, equal, even, firm, fixed, flat, flinty, frozen, grim,
   hard, hard-core, homogeneous, immobile, immovable, immutable,
   implacable, impliable, inalterable, incommutable, inconvertible,
   indefeasible, inelastic, inert, inexorable, inflexible,
   insusceptible of change, intact, intransigent, intransmutable,
   invariable, inviolate, iron, irreconcilable, irretrievable,
   irreversible, irrevocable, lasting, level, measured, mechanical,
   methodic, monolithic, noble, nonreturnable, nonreversible,
   of a piece, ordered, orderly, permanent, perpetual, persistent,
   quiescent, regular, relentless, remaining, reverseless, rigid,
   rigorous, robotlike, rock-ribbed, smooth, solid, stable, static,
   stationary, staying, steadfast, steady, steely, stern, stiff,
   sustained, systematic, torpid, unaffected, unalterable,
   unalterative, unaltered, unbending, unbroken, unchanged,
   unchanging, unchecked, uncompromising, undeflectable, undestroyed,
   undeviating, undifferentiated, undiversified, unfading, unfailing,
   ungiving, uniform, unmodifiable, unmovable, unmoved, unrelenting,
   unremitting, unrestorable, unreturnable, unruffled, unshifting,
   unsusceptible, unvariable, unvaried, unvarying, unyielding