Converting varchar() to text

Is there a safe way to convert varchar(n) to text, other than create
a new column, update, delete column, rename?

I have a number of databases that were built with varvhar(n) and which
should have been done with text. They're in production, and I'd rather
not take the downtime needed to convert some rather large tables - the
bulk update hitting every row of the large table makes it effectively
unvacuumable, and vacuum full requires locks that effectively shut
down the entire system.

They're the same format on disk, so I'm guessing that some diddling
with pg_attribute may be possible. Does anyone have any experience
doing this?

Cheers,
  Steve