now it's working


include/mysql.h:
  read_change_user_result 'virtual' method added
libmysql/client_settings.h:
  cli_read_change_user_result interface
libmysql/libmysql.c:
  cli_read_change_user_result implementation
libmysqld/lib_sql.cc:
  emb_read_change_user_result implementation
sql-common/client.c:
  cli_read_change_user_result added to the method's table
sql/sql_parse.cc:
  fixes to make mysql_change_user working in embedded library