CREATE TABLE #Transfer
(
	ID INT IDENTITY PRIMARY KEY,
	OldKey INT,
	NewKey INT
)

INSERT INTO #Transfer (OldKey, NewKey)
VALUES (1, 2)
INSERT INTO #Transfer (OldKey, NewKey)
VALUES (2, 3)
INSERT INTO #Transfer (OldKey, NewKey)
VALUES (3, 4)
INSERT INTO #Transfer (OldKey, NewKey)
VALUES (5, 6)
INSERT INTO #Transfer (OldKey, NewKey)
VALUES (6, 7)

ID	OldKey	NewKey
1	1	2
2	2	3
3	3	4
4	5	6
5	6	7

SELECT 1 AS OldKey, 4 AS NewKey
UNION
SELECT 2 AS OldKey, 4 AS NewKey
UNION
SELECT 3 AS OldKey, 4 AS NewKey
UNION
SELECT 4 AS OldKey, 4 AS NewKey
UNION
SELECT 5 AS OldKey, 7 AS NewKey
UNION
SELECT 6 AS OldKey, 7 AS NewKey
UNION
SELECT 7 AS OldKey, 7 AS NewKey

OldKey	NewKey
1	4
2	4
3	4
4	4
5	7
6	7
7	7

CREATE TABLE #Transfer
(
	ID INT IDENTITY PRIMARY KEY,
	OldKey INT,
	NewKey INT
)

INSERT INTO #Transfer (OldKey, NewKey)
VALUES (1, 2)
INSERT INTO #Transfer (OldKey, NewKey)
VALUES (2, 3)
INSERT INTO #Transfer (OldKey, NewKey)
VALUES (3, 4)
INSERT INTO #Transfer (OldKey, NewKey)
VALUES (5, 6)
INSERT INTO #Transfer (OldKey, NewKey)
VALUES (6, 7)

;WITH Transfers AS (
    SELECT Oldkey=Newkey, Newkey
    FROM #Transfer
    UNION ALL
    SELECT a.Oldkey, a.Newkey 
    FROM #Transfer a
    UNION ALL
    SELECT b.Oldkey, a.Newkey
    FROM Transfers a
    JOIN #Transfer b ON b.NewKey = a.Oldkey
    )
SELECT OldKey, NewKey=MAX(NewKey)
FROM Transfers
GROUP BY OldKey
ORDER BY OldKey, NewKey

DROP TABLE #Transfer