Updating a table

Hi,

Release 11.2.0.3.0

I have to copy one column value to another column in the same table. There is a WHERE clause condition for which I have to do so. For rest have of the data, I need not to touch . There is around 250 million records that I have to update. I know if it would have been without any condition, a DDL(CTAS query) is better. I am afraid of using UPDATE query, as it need more UNDO space and other resources.

Please let me know whats the best way to do it.

Thanks,

Vikash