Hi
I don't know, if my reply was clear. If you create your table as
CREATE TABLE T1 ... COL1 SMALLINT NOT NULL, then you cannot change the null constraint, because DB2 does not offer this feature.
If you create your table with CREATE TABLE T1 ... COL1 SMALLINT CHECK (COL1 IS NOT NULL) then you can drop this check constraint, by using ALTER TABLE T1 DROP constraintname.