Yeah, to clarify, AC will never sell a seat to a revenue passenger if it would result in an overbooked J cabin. That does not mean they never overbook J.

For PY, I don't know why the policy would apply. If a flight is J9 O0, and you can get the O cash but not the J cash, why not just take it and upgrade someone. You never want to have to downgrade someone, but overbooking PY has the same consequences as overbooking Y, so it seems reasonable to do it.