Rickey Henderson is replacing Rick Down as hitting coach of the Mets starting Thursday. Henderson, who had been a special instructor for the Mets this season, is a 10-time All-Star who set career records for stolen bases (1,406) and runs (2,295). He was the AL MVP in 1990. The Mets' decision to make Henderson their hitting coach was first reported Wednesday night by Newsday on its Web site.