Renaming a constraint

I found an old post regarding the subject, where modifying the
pg_constraint entry was recommended:

http://archives.postgresql.org/pgsql-admin/2003-04/msg00339.php

Is this still safe to do ? The pertinent docs don't say anything pro or
contra:
http://www.postgresql.org/docs/8.2/static/catalog-pg-constraint.html

I need to rename a few constraints to keep our schema logically
consistent. It would be of course nice to have an ALTER TABLE option to
do it, but if updating pg_constraint is safe it would do it for now.

Thanks,
Csaba.