WordNet sense 1 (an unfortunate happening that hinders or impedes; something that is thwarting or frustrating):

blow, reverse, setback, black eye

WordNet sense 2 (turning in an opposite direction or position):

turn around

WordNet sense 3 (a decision to reverse an earlier decision):

flip-flop, turnabout, turnaround, change of mind

WordNet sense 4 (turning in the opposite direction):

reversion

WordNet sense 5 (the act of reversing the order or place of):

transposition

WordNet sense 6 (a major change in attitude or principle or point of view):

about-face, volte-face, policy change