altering foreign keys

In my database, I have a core table that nearly all other tables
key against.  Now I need to adjust all of those foreign keys to
add a "on update cascade" action.  Is there a way to alter the
existing keys? (it didn't jump out at me in the manual)

If not, is there a serious issue preventing this feature?

If I have to drop and re-create all of the foreign keys, is it
possible to wrap the whole operation in a transaction without
risking invalid inserts in the referring tables? (I come from
an Oracle background, where DDL causes an implicit commit)