Alter columnt type from int to bigint with minimal downtime

I have several tables with id columns that are declared to be serial (i.e. integer type with default nextval()). Converting them to bigint seem to be quite straightforward:

ALTER TABLE my_table
ALTER COLUMN id
TYPE bigint;

But that might take a lot of time and lock the whole table. It's okay-ish solution for cases when the whole site is already down due to integer overflow, but not so good for provisional changes. How would one do that concurrently?