Speed of conversion from int to bigint

Hi,
(Postgres 9.5 and 9.6)
We have a table of about 650million rows. It's a partitioned table, with two "child" tables. We want to change its primary key type from int to bigint while retaining the current values.

We're using this:

ALTER TABLE dta.my_table ALTER column table_id TYPE bigint;

But it's taking a very long time, and locking the database. We're going to need to do this in production as well, so a long-term table-lock isn't workable.
Is there anything we can do to speed things up? How long is this likely to take?

Thanks,
Jonathan