ALTER TABLE <tablename> ALTER COLUMN <columnname> SET DATA TYPE BIGINT
will not do the job?
I've found some varchar columns in a couple of tables too small and used the above to increase their size. Worked perfectly.
Something else noteworthy is that with varchar, there is no rewrite of the table. You are just removing or loosening the length restriction on a variable width column type. I believe you could change all columns from VARCHAR(n) to TEXT or VARCHAR(n+X) and have it take almost no time at all since you are only impacting the catalog tables (pretty sure about that at least). With a fixed width column like int4 to int8, all the rows need to be actually re-written.