On what date was James P. Buchanan (d)'s successor seated?
CREATE TABLE "table1_2159547_3" (
  "district" text,
  "vacator" text,
  "reason_for_change" text,
  "successor" text,
  "date_successor_seated" text
);
SELECT "date_successor_seated" FROM "table1_2159547_3" WHERE "vacator"='James P. Buchanan (D)';