J.P. Morgan Chase has launched syndication of a $400 million, two-year revolving credit for The Great Atlantic & Pacific Tea Company. The facility, priced at LIBOR plus 21/2%, will replace an existing $500 million, five-year facility also led by Chase Manhattan Bank. Officials at A&P declined to comment on banking arrangements. Calls to the banks were not returned by press time. A Chase spokeswoman declined to comment.