No, the FTTC upgrade will not directly* improve the performance of existing (ADSL) broadband connections. These will continue to use only the copper cables all the way to Cranleigh exchange. To take advantage of FTTC your line needs to be linked to the new fibre optic cable at the cabinet. Lines using FTTC will also remain connected to the old copper wire to Cranleigh for voice calls. Not surprisingly then, the new VDSL (FTTC) service will cost significantly more than an equivalent ADSL service in most circumstances (for example, Plusnet are charging £10 per month more). Also note that, as far as we can see, BT are only providing for up to 100 fibre connections per cabinet, so it appears impossible to upgrade all ADSL broadband connections anyway (without further engineering works on the new cabinets). This seems strangely short sighted, given that adequate capacity for all would cost little more.
* You may see some indirect improvement due to a reduction in “congestion” if others sharing your ADSL bandwith move over to VDSL. You could alternatively see some degradation in your ADSL service if a VDSL connection interferes in some way (e.g. cross-talk). Also bear in mind that an engineer needs to break and remake each connection which is upgraded, so we can expect to see some existing connections struggling as a result of this disturbance.