The switch to LLU is an actual physical cable change whereas switch within BTW is just a software change. It's possible that BT made the physical change to your line but that Zen hadn't yet done their bit to bring you onto their network - potentially nothing in PlusNet's control at all.
As the original poster is on FTTC, it's just a software reconfiguration of the Openreach systems to present traffic from the customer's FTTC DSLAM port to the Zen C-VLAN at the handover point rather that the BT Wholesale C-VLAN.
I made the transition from BT Wholesale to Zen LLU last year whilst a Zen FTTC customer because Zen unbundled my exchange. The connection dropped on BT Wholesale at 00:21 and came back up on Zen LLU at 00:33. Obviously there was no change in PPPoE user name and password involved, as I remained a Zen customer.
I thought Zen had MSANs in at least some exchanges though I appreciate there is some doubt as to exactly what Zen install. Unless things have changed, Zen always use WLR3 for voice rather than a voice line card in a Zen MSAN.