Probably just based on loads. If it was scheduled like that for a while - which would seem likely - there wouldn't be any premium economy passengers as it would not have been sold. If it is only a recent change, the W passengers would be either upgraded to J (for free) or downgraded to Y (and entitled to EC261 downgrade payment) or in all likelihood (though this is less of a formal entitlement) allowed to move to a different date/service with W offered (or F for F pax).