Chronicle of a Changing City.

THE PARK AVENUE CHRISTMAS LIGHTS are a tradition that goes back to 1945, when a group of neighborhood families arranged to put up majestic fir trees on the avenue's central malls to honor soldiers who died in World War II. The group turns on the lights on the first Sunday of December, after a festive ceremony outside the Brick Presbyterian Church at 91st Street. Early on, the cost was paid by the founding families. Nowadays, the $10,000 tab is picked up by the Fund for Park Avenue, which also maintains the mall plantings, and whose lighting committee includes Muffie Potter, David Rockefeller and Joanne Woodward, said the fund's president, Barbara McLaughlin. Ms. McLaughlin said the trees and lights, which run from 54th to 96th Street, remain a memorial to ''all who have lost their lives defending our country.'' Each of the 100 trees is adorned with 2,500 little stringed lights, which are plugged into the bases of streetlamps. They remain up through mid-January. [ABSTRACT FROM AUTHOR]