alter column type

I have a database in which one table references the primary key of
another. The type of the primary key was initially int, but I changed it
to bigint. However, I forgot to update the type of a column that
references it. So, I've initiated "ALTER TABLE foo ALTER COLUMN bar TYPE
bigint", where foo/bar is the table/column referencing the primary key that
is now of type bigint.

However, with 2^31 rows, it is taking a "long" time to write the rows (it's
been 12 hours). Is there a more efficient way to do this? Even if/when
this one finishes, there are other column types that I have to update.
This update effectively locked me out of all access to the data anyway, so
I don't foresee any concern of writes that might affect integrity.

Cheers,
Casey