dissimilate

from WordNet (r) 3.0 (2006)

dissimilate
    v 1: become dissimilar by changing the sound qualities; "These
         consonants dissimilate" [ant: {assimilate}]
    2: make dissimilar; cause to become less similar [ant:
       {assimilate}]
    3: become dissimilar or less similar; "These two related tribes
       of people gradually dissimilated over time" [ant:
       {assimilate}]

from The Collaborative International Dictionary of English v.0.48

Dissimilate \Dis*sim"i*late\, v. t.
   To render dissimilar.
   [1913 Webster]