Erm, they normally just remove the old shell and fit the new shell on top, this giving the required space.
Saying that though we have both an
old PCP Cabinet as well as a new one, still not sure if anything has been moved over to it, possibly has due to our connection is more stable than it used to be, not perfect, but better.
Our road is split up between 2 cabinets, the PCP cabinet for the top half was already a new cabinet that caters for 543 homes (FTTP: 304, ADSL: 239), however the lower half including our home was using the old tatty PCP cabinet catering for about 400 homes (all ADSL) and was installed a new PCP cabinet about 2 years ago.
But yeah, if needed they can just change the outer shell to a much larger one.
You might be lucky and might get the new Tool less PCP Cabinet, they look nice and neat and tidy inside
Paul