Quickly change NULL column to NOT NULL

@ypercube's answer does manage this partially as a metadata only change.

Adding the constraint with NOCHECK means that no rows will need to be read to verify it, and if you are starting from a position where the column does not contain NULL values (and if you know none will be added between checking and adding the constraint) then, as the constraint prevents NULL values being created from future INSERT or UPDATE operations, this will work.

Adding the constraint can still have an impact on concurrent transactions however. The ALTER TABLE will need to acquire a Sch-M lock first. Whilst it is waiting for this all other table accesses will be blocked as described here.

Once the Sch-M lock is acquired the operation should be pretty quick however.

One problem with this is that even if you know the column in fact has no NULLs the constraint is not trusted by the query optimiser which means that the plans can be sub optimal.

CREATE TABLE T (X INT NULL)

INSERT INTO T 
SELECT ROW_NUMBER() OVER (ORDER BY @@SPID)
FROM master..spt_values

ALTER TABLE T WITH NOCHECK
  ADD  CONSTRAINT X_NOT_NULL 
    CHECK (X IS NOT NULL) ; 

SELECT *
FROM T 
WHERE X NOT IN (SELECT X FROM T)

Compare this with the simpler

ALTER TABLE T ALTER COLUMN X INT NOT NULL

SELECT *
FROM T 
WHERE X NOT IN (SELECT X FROM T)

One possible problem you might encounter with altering the column definition in this way is that it not only needs to read all the rows to verify that they meet the condition but also can end up actually performing logged updates to the rows.

A possible half way house might be to add the check constraint WITH CHECK. This will be slower than WITH NOCHECK as it needs to read all rows but it does allow the query optimiser to give the simpler plan in the query above and it should avoid the possible logged updates issue.

You could, instead of altering the column, add a table CHECK constraint with the NOCHECK option:

ALTER TABLE MyTable WITH NOCHECK
  ADD  CONSTRAINT MyColumn_NOT_NULL 
    CHECK (MyColumn IS NOT NULL) ;