  We study how Gromov-Witten invariants of projective 3-folds transform under a
standard flop and a small extremal transition in the algebro-geometric setting
from the recent development of algebraic relative Gromov-Witten theory and its
applications. This gives an algebro-geometric account of Witten's wall-crossing
formula for correlation functions of the descendant nonlinear sigma model in
adjacent geometric phases of a gauge linear sigma model and of the symplectic
approach in an earlier work of An-Min Li and Yongbin Ruan on the same problem.
A terse account from the stringy and the symplectic viewpoint is given in the
appendix to complement and compare to the discussion in the main text.
