Source : Webster's Revised Unabridged Dictionary (1913)

Commutable \Com*mut"a*ble\, a. [L. commutabilis.]
   Capable of being commuted or interchanged.

         The predicate and subject are not commutable.
                                                  --Whately.

Source : WordNet®

commutable
     adj 1: subject to alteration or change; "the death sentence was
            commutable to life imprisonment" [ant: {incommutable}]
     2: capable of being exchanged for another or for something else
        that is equivalent [syn: {substitutable}]