To get node-level vectoring, which is probably needed, the current Huawei cabinets will need new backplanes that are capable of transferring the vectoring data between cards. That's the way I read the Huawei specs, anyway.
That means the card-cage holding the active DSLAM electronics needs to be dismantled, although it is probably more likely that a swap-out - swap-in process would be used, to minimise downtime. It is still a longer process than just adding cards, or changing connectors on the front panel.
But who knows: the trials that BT are undertaking/extending might be trials of different deployment options, to see if they can minimise the need for swapping hardware.