To remove tight cord, cut wire off about 12 inches from lamp's base, slit cord's two conductors apart, and strip about an inch of insulation off ends. Do the same to one end of new length of cord.
Twist bare new and old conductor ends together, and fold twists flat along cord. Wrap electrical tape around splice. Pull on old cord from top of fixture, and work new cord through; at same time, push on new cord from bottom. When you have sufficient length of new cord through top, clip off old cord.