Adding one or more fields to an existing ODS

Trying to understand the steps BW (3.x) goes through when we alter an existing ODS structure to include a couple new fields (Oracle 9i is the underlying database).

Business rules state that the fields only need to be available on a go forward basis (i.e. no data needs to be updated on the 'old' records).

From current experience, it appears that the ODS structure is unloaded to temp table (maybe? was not clear), the structure is altered, and all records that exist are issued an update set command that sets the new field(s) to default or nulls.

Is this accurate? If so, is there an alternative to issuing such an update (time to perform is the issue). If there is no alternative, is it fair to say that once we create an ODS (or series of partitioned ODS's) and they fill with data over time, if we ever want to add a field or two going forward, we will have a very large conversion effort to contend with?

Obviously, we are hoping there is the equivalent of an alter table command (with nulls), rather than a large update set of commands (caused by an alter table statement with defaults).