Changing a not-null column to null

I know there is a DDL-based technique to change a not-null column to null, discussed  in the Mckoi mailling list. One problem with this approach is that an  'alter create table' drops all constraints on the table, and they will  have to be recreated. Is there a way to achieve this effect via DML? In  other words, is there some system table that we can update to make one  of the user table columns nullable? If so, please explain.
  
  Thanks in advance.
  

M. A. Sridhar 
m_a_sridhar at yahoo dot com