How to delete default privileges

I have a DB where we changed ownership of all objects.

We had:

ALTER DEFAULT PRIVILEGES FOR ROLE old_dbowner GRANT SELECT ON tables
TO dbgroup_ro_group;
ALTER DEFAULT PRIVILEGES FOR ROLE old_dbowner GRANT SELECT ON sequences
TO dbgroup_ro_group;

ALTER DEFAULT PRIVILEGES FOR ROLE old_dbowner GRANT SELECT, UPDATE,
DELETE, INSERT ON tables TO dbgroup_rw_group;
ALTER DEFAULT PRIVILEGES FOR ROLE old_dbowner GRANT SELECT,
USAGE ON sequences TO dbgroup_rw_group;
ALTER DEFAULT PRIVILEGES FOR ROLE old_dbowner GRANT
EXECUTE ON functions TO dbgroup_rw_group;

But now there is a new DB owner and I have run the above, but with
new_dbowner. How can I delete the old default grants? Can't find
examples and don't see it on the documentation either.

Any help would be greatly appreciated.