BENEFITS FOR THE HOMELESS are planned for this weekend. In Boston, 12 hours of nonstop rock music will be presented Saturday at the Bayside Exposition Center by performers who include X, November Group, Rick Berlin - the Movie, and Cheap Trick. In New York, an 8-hour concert Sunday at Columbia University will feature Lionel Hampton, Lester Bowie, Gil Evans, and others. RAYMOND LEPPARD will take the helm at the Indianapolis Symphony Orchestra next September. The British-born conductor, whose credits include guest appearances with the Chicago Symphony, New York Philharmonic, and Boston Symphony, will also continue as principal guest conductor for the St. Louis Symphony. He replaces John Nelson. THE BOSTON SYMPHONY ORCHESTRA has named Kenneth Haas as its new managing director. In March, Mr. Haas will leave a similar post with the Cleveland Orchestra to replace Thomas W. Morris, who resigned from the BSO last January.