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):

interchangeably
    adv 1: in an interchangeable manner; "these terms can be used
           interchangeably"

Bouvier's Law Dictionary, Revised 6th Ed (1856):

INTERCHANGEABLY. Formerly when deeds of land were made, where there Were
covenants to be performed on both sides, it was usual to make two deeds
exactly similar to each other, and to exchange them; in the attesting
clause, the words, In witness whereof the parties have hereunto
interchangeably set their hands," &c., were constantly inserted, and the
practice has continued, although the deed is, in most cases, signed by the
grantor only. 7 Penn. St. Rep. 320.