WordNet (r) 3.0 (2006):

go back on
    v 1: fail to fulfill a promise or obligation; "She backed out of
         her promise" [syn: renege, renege on, renegue on, go
         back on]

Moby Thesaurus II by Grady Ward, 1.0:

32 Moby Thesaurus words for "go back on":
   abandon, back out, be unfaithful, beg off, break faith, cry off,
   depart from, desert, discard, drop out, evacuate, fail, forsake,
   jettison, jilt, leave, leave behind, leave flat, maroon,
   pass the buck, pull out, quit, quit cold, renege, say goodbye to,
   shift the blame, shift the responsibility, stand down,
   take leave of, throw over, vacate, withdraw