I found this article: Vectored DSL to the Rescue
It gives a description in something slightly below degree-level physics, but has this to say on the topic of mixing the old & new technology:
Implication #3: Coexistence Among Vectored and Non-Vectored DSL Lines
It is possible that non-vectored and vectored lines share the same binder; for example, if legacy DSLAMs are not replaced as newer vectored DSLAMs are installed. In such cases, the crosstalk generated from the non-vectored lines to the vectored lines cannot be cancelled. If left unmanaged, such crosstalk will reduce the benefits of crosstalk cancellation performed among the vectored lines. The proper management practice is then to reduce the transmitted power levels of the non-vectored lines to a level no higher than what is the minimum required to maintain their service requirements. Any such adjustment of the transmitted power levels must be made on a line-by-line basis, after taking into account the conditions under which each line operates.
Somehow, I don't think Ofcom will let BT reduce power on all their competitior's DSLAMs ...
It still looks to be a beautiful technology boost - a real no-brainer that reinforces justification of the whole FTTC strategy - and might (because it extends the useful life of an FTTC cabinet) make more rural deployments financially viable. On the other hand, it might make some suburban FTTP
It would be a crying shame if the structure of the UK market meant it couldn't be employed.