The point here is, is there a way to do this through DML only. Looks like the suituation here is, in production system we need to cleanup the data but we don have the acces to create DDL.
The solution posted by akolla is taken from here:
If you cannot use 'ALTER TABLE' you could use the identifier column ('X') as the 'flag' field if you can find a range of IDs not used (f. ex.: negative numbers, multiply x some value, etc...)