The Collaborative International Dictionary of English v.0.48:

Interchangeable \In`ter*change"a*ble\, a. [Cf. OF.
   entrechangeable.]
   [1913 Webster]
   1. Admitting of exchange or mutual substitution.
      "Interchangeable warrants." --Bacon.
      [1913 Webster]

   2. Following each other in alternate succession; as, the four
      interchangeable seasons. --Holder. --
      In`ter*change"a*ble*ness, n. -- In`ter*change"a*bly,
      adv.
      [1913 Webster]

WordNet (r) 3.0 (2006):

interchangeable
    adj 1: (mathematics, logic) such that the arguments or roles can
           be interchanged; "the arguments of the symmetric
           relation, `is a sister of,' are interchangeable"
    2: capable of replacing or changing places with something else;
       permitting mutual substitution without loss of function or
       suitability; "interchangeable electric outlets"
       "interchangeable parts" [syn: exchangeable,
       interchangeable, similar, standardized, standardised]

Moby Thesaurus II by Grady Ward, 1.0:

59 Moby Thesaurus words for "interchangeable":
   alike, assignable, changeable, communicable, commutable,
   commutative, conductive, consignable, contagious, convertible,
   conveyable, equal, equalizing, equivalent, even, exchangeable,
   exchanged, expressable, give-and-take, impartable, indiscernible,
   indistinct, indistinctive, indistinguishable, interchanged,
   mailable, metathetic, movable, mutual, permutable, portable,
   portative, reciprocal, reciprocating, reciprocative, removable,
   retaliatory, returnable, standard, stereotyped, substitutable,
   swapped, switched, traded, transferable, transfusable,
   transmissible, transmissive, transmittable, transportable,
   transportative, transposable, transposed, undifferentiated,
   undiscriminated, undistinguishable, undistinguished, uniform,
   without distinction