SWEEPING CHANGES PUSHED FOR CODE ON CITY HIGH-RISES
New York City officials consider sweeping plan to change codes that govern how all high-rise buildings are protected against fire and structural damage and how they are evacuated in emergency; plan is one of first clear signs that fiery collapse of World Trade Center is going to transform construction and operation of high-rises around nation; possible changes would be among city's most comprehensive in three decades; changes are proposed by task force of building and fire officials, real estate leaders, engineers and construction executives; great loss of life in World Trade Center disaster is creating impetus for initial wave of changes that safety experts have long supported; task force's ideas will be aired in daylong public hearing on Aug 13; goal is to have completed plan ready by end of year, for swift consideration by City Council and Mayor Michael R Bloomberg; diagram illustrating suggested changes (M)