altering foreign key without a table scan

Hi list,

as part of a db schema update, I'd like to alter the "on update" property of a
fkey, for example going from :
> ALTER TABLE ONLY foo ADD CONSTRAINT foo_barid_fkey FOREIGN KEY (barid)
> REFERENCES bar(id) ON UPDATE CASCADE ON DELETE CASCADE;
to :
> ALTER TABLE ONLY foo ADD CONSTRAINT foo_barid_fkey FOREIGN KEY (barid)
> REFERENCES bar(id) ON UPDATE RESTRICT ON DELETE CASCADE;

I understand I can create the new fkey and drop the old one, but this requires
a scan of the table (to check that no existing data violates the new fkey)
which, on this large, heavily-updated, no-downtime table I can't really aford.

The thing is, I know there is no violation by existing data, because of the
existing fkey. So locking and scaning the table to add the "duplicate" fkey is
not necessary. In a sense, I'm looking for :
> ALTER TABLE foo ALTER CONSTRAINT foo_barid_fkey ON UPDATE RESTRICT;
I'm guessing/wishfull-thinking that some hackery with the system catalog could
emulate that ?

I'm currently using postgres 8.3 (we want to upgrade, but it's hard to
schedule).

Thanks in advance.

--
Vincent de Phily