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,