change username

I have a user on my system whose username has recently changed.  The user
owns and is a non-owning user of multiple database.  I have tried a command
of the form:

update pg_user set usename='newname' where usesysid=###;

When I try this I get an error.  Does anyone know how to change this
information?  If I delete the user and recreate the user with the same uid
does that retain all the existing database permissions?

--
Bill Brandt
brandtwr@draaw.net                              http://www.draaw.net/