I'm not sure understand you correctly, but..
Below query replaces
NULL
in
UserField
with
DM_ID
values.
UPDATE t1 SET t1.Userfield = t2.DM_ID, t1.DM_ID = t2.AM_ID
FROM dbo.Account AS t1 INNER JOIN Account AS t2 ON t1.Acct_ID = t2.Acct_ID
Fully working sample:
DECLARE @Account TABLE (Acct_ID INT, AM_ID INT, DM_ID INT, Userfield INT NULL)
INSERT INTO @Account (Acct_ID, AM_ID, DM_ID)
VALUES(111, 1, 2),
(123, 3, 4),
(321, 5, 6)
SELECT *
FROM @Account
UPDATE t1 SET t1.Userfield = t2.DM_ID, t1.DM_ID = t2.AM_ID
FROM @Account AS t1 INNER JOIN @Account AS t2 ON t1.Acct_ID = t2.Acct_ID
SELECT *
FROM @Account
Results:
1. query
111 1 2 NULL
123 3 4 NULL
321 5 6 NULL
2. query
111 1 1 2
123 3 3 4
321 5 5 6