The Collaborative International Dictionary of English v.0.48:

Modify \Mod"i*fy\, v. t. [imp. & p. p. Modified; p. pr. & vb.
   n. Modifying.] [F. modifier, L. modificare, modificari;
   modus limit + -ficare (in comp.) to make. See Mode, and
   -fy.]
   1. To change somewhat the form or qualities of; to change a
      part of something while leaving most parts unchanged; to
      alter somewhat; as, to modify a contrivance adapted to
      some mechanical purpose; to modify the terms of a
      contract.
      [1913 Webster]

   2. To limit or reduce in extent or degree; to moderate; to
      qualify; to lower.
      [1913 Webster]

            Of his grace
            He modifies his first severe decree.  --Dryden.
      [1913 Webster]

WordNet (r) 3.0 (2006):

modify
    v 1: make less severe or harsh or extreme; "please modify this
         letter to make it more polite"; "he modified his views on
         same-gender marriage"
    2: add a modifier to a constituent [syn: modify, qualify]
    3: cause to change; make different; cause a transformation; "The
       advent of the automobile may have altered the growth pattern
       of the city"; "The discussion has changed my thinking about
       the issue" [syn: change, alter, modify]

Moby Thesaurus II by Grady Ward, 1.0:

111 Moby Thesaurus words for "modify":
   abate, accommodate, adapt, adjust, adjust to, alter, ameliorate,
   amend, analyze, anatomize, assuage, atomize, better, box in,
   break up, change, chop logic, circumscribe, condition, convert,
   decrease, deform, denature, desynonymize, difference,
   differentiate, diminish, discriminate, disequalize, disjoin,
   distinguish, diversify, divide, fit, hedge, hedge about, improve,
   individualize, individuate, leaven, lessen, limit, lower,
   make a distinction, mark, mark off, mark out, meliorate, mitigate,
   moderate, modulate, mutate, narrow, overthrow, palliate,
   particularize, personalize, qualify, re-create, realign, rebuild,
   reconstruct, redesign, redo, reduce, refashion,
   refine a distinction, refit, reform, regulate by, remake, remodel,
   renew, reorganize, reorient, reshape, restrain, restrict,
   restructure, revamp, revise, revive, reword, rework,
   ring the changes, season, segregate, separate, set apart,
   set conditions, set limits, set off, sever, severalize,
   shift the scene, shuffle the cards, soften, specialize,
   split hairs, subvert, temper, tone down, transform, turn,
   turn the scale, turn the tables, turn the tide, turn upside down,
   vary, work a change, worsen