As Chrysalis says, not going from BT 80/20 to PlusNet 80/20 or PlusNet 40/20.
My modem remained synced through the transfer process, only the router had to be touched.
DLM usually works pretty well (if slowly) and if your line is now good should restore your speed. Even if you do get a DLM reset but the line is still bad, any increase you get from the DLM reset will soon be taken away again.
Recently moved from BT Infinity 2 to PlusNet. Very happy so far.