Worrying about 1 billion SSNs isn't a problem with the storage arrangement: Oracle will allow you to alter the table to widen the column. alter table OMG_BTW modify (SSN varchar2(10)); will do exactly that.

The problem you were having yesterday was due to the fact that you were trying to change the underlying datatype from varchar2 to number. You can't do that if there's any data in the column (I'm not sure you can actually do it even then). Similarly, you can't make a column narrower if there's any data in it (even if the data doesn't require the full width of the column). To perform such an act, you must ensure that all the values are null.

The problem is if you start making assumptions in other places about the size of the data values: either declaring enough storage to only hold 9 digits, or to only display 9 digits. These will bite you hard when you suddenly need to squeeze that extra numeral in. And it will be sudden, no matter how much warning you had ;-)

As to what type you should use, personally, I'd declare it as number(9, 0) to minimize the risk of getting invalid values in there, and reduce the amount of storage required. After all, you think it's storing a nine digit number, don't you? (Unless, as tye mentioned, you ought to be allowing for foreign values).

--
Tommy
Too stupid to live.
Too stubborn to die.