Some additional information

I have been able to downgrade to get on an earlier flight on a standby basis on the original confirmed day of travel even when L or D was not available. We all know that the rules for standby on any airline is what the gate agent allows you to do so no generalities can be drawn from that.

For advanced confirmed booking or re-booking I have many times been not allowed to fly in J on an AONE if no D was available and have also not been able to re-ticket to Y on a DONE if L was not available.

I can imagine that some travelers have seen exceptions. I have benefited from exceptions/mistakes on OW tickets also (flying on a TP code-share booked under the IB flight number and several times having the 10 day minimum trip length waved). The rules are the rules though even if they don't make sense.