Increasing the length of a VARCHAR column without table rewrite

ALTER TABLE .... ALTER column TYPE varchar(xxx)

We need to increase the length of a varchar(256) column in two of our
largest tables preferrably to TEXT.

Is there a way to do this without having to go through a complete table
rewrite?

I have googled and seen the todo on the wiki, but I'm wondering if we are
increasing the length of a varchar column can we manually modify the system
catalogs to increase the length?

http://wiki.postgresql.org/wiki/Todo#ALTER

Version: PG 8.1.17

Thanks,
Eric