Cut one pear in half lengthwise. Cut a ⅛-inch slice from one of pear halves. Coat a 1-quart glass soufflé dish with cooking spray; line bottom of dish with wax paper. Center pear slice in bottom of dish. Trim crusts from bread; discard crusts. Cut one bread slice into small cutes; set aside. Cut 5 bread slices in half lengthwise. Standing bread slices vertically in dish, line sides with 10 bread halves. Place 1 bread slice in bottom of dish; arrange bread cubes lightly around whole bread slice.
Peel and core remaining pears; cut lengthwise into slices to equal 8 cups, and set aside.
Melt margarine in a large skillet over medium heat. Stir in sugar, lemon juice, and cinnamon. Add pears; stir gently. Cover and cook pears 15 minutes, stirring occasionally. Uncover and cook pears an additional 15 minutes. Drain pears, reserving liquid.
Spoon pear mixture into dish; press firmly with the back of a spoon. With scissors, trip tops of bread halves even with pears; reserve trimmed bread tops. Dip remaining whole bread slice and reserved trimmed bread tops in reserved pear liquid; arrange in center of dish over pears. Cover bread completely with aluminum foil, and cut 6 1-inch slits in foil. Bake at 350 degrees for 50 minutes or until lightly browned. Uncover and cool in dish 1 hour on a wire rack. To serve, loosen edges of charlotte with a knife; invert charlotte onto a plate. Combine frozen yogurt with reserved pear juice and serve with pear charlotte.