just a recent observation of a DM on regional flights. Based on accounts of flight with cabin sold as PE (so 3 class, J, PE, Y). the KVS looked like this, meaning decent load in J cabin and somewhat light PE and overbooked Econ.
F9 A9 J5 C5 D5 I4 W7 R7 E5 Y0 B0 H0 K0 M0 L0 V0 S0 N0 Q0 O0
He was initially moved to PE seat T-10 hours (we kept checking online checkin page)... T-3 hours he was subsequently moved to J cabin. According to belief of "one-class upgrade only", it would have meant entire PE pax were moved to J before my DM friend (with W7R7E5 type of load, it suggest at least 10-15 occupied paid PE pax) was able to score a "double upgrade" from Y to J. But I have different theory...
I think, strictly for purpose of op-ups whether regional/long-haul CX will look at PE and Economy as a same cabin class... so first thing they do is jiggle around and move economy DM/Golds to PE seats like they would always do... then when the entire PE+Y cabin combined together is full and requires an "op-up", they will select the lucky guy from the combined pack, treating as one large cabin. Thus a Gold in PE holding W fare would rank after DM on V-class. This also consistent with previous observation on a SYD route when DM were put straight to J class from Y cabin on a full PE/Y cabin.
Summary (awaiting other case study)
if DM is to travel on Y0 B0 H0 K0 M0 L0 V0 S0 N0 Q0 O0 type of a flight, don't bother booking PE for hoping of op-up, just force yourself in with V-class gurantee and let them pick you to sit at J ahead of others holding PE.
(this guy scored double upgrade on both inbound/outbound for his weekend getaway)