Postgres insert or update trigger WHEN condition (old)

I need write insert or update trigger, but with WHEN condition with compare OLD and NEW rows.

According documentation OLD is null for insert operation. How i can use OLD in WHEN condition for INSERT AND UPDATE triggers?

Example trigger:

CREATE TRIGGER mytrigger
    BEFORE INSERT OR UPDATE ON "mytable"
    FOR EACH ROW 
    WHEN (NEW.score > 0 AND OLD.score <> NEW.score)
    EXECUTE PROCEDURE mytrigger();

but for insert OLD is null.