[edited March 2015 at the end to include info regarding SDH bypass. Please also see my post of 23 March 2015 for more info regarding the SDH bypass]
[Edit Dec 15 - Ledaig exchange is NOT being upgraded to 21CN for FTTC backhaul - the fibre comes up from Oban. Plans to upgrade the exchange to 21CN are unchanged - i.e. no planned date]
IF the exchange is upgraded to 21CN WBC, then there is the possibility that existing ADSL connections that choose not to migrate to FTTC will still be migrated to the new 21CN BT link anyway, in order that the old '20CN' network can be retired. This won't affect your current ADSL connection or voice service.
If this is the case, then it is possible that the equipment in the exchange is upgraded to allow connections using ADSL2+
IF this were the case, then end users on Exchange-only lines, or those who do not wish to take up FTTC connections, can benefit from ADSL2+ speeds (IF you are within around 4km of the exchange) and stability over their existing ADSLMax connection.
However, there are many unknowns still, and I reach the end of my current knowledge:
1) Just because the exchange is being upgraded to 21CN WBC in order to handle the new FTTC traffic, doesn't mean that they will automatically install new ADSL2+ MSAN/DSLAM in the exchange as well. This used to be the case before FTTC - an exchange would simultaneously receive 21CN and ADSL2+ compatible MSANs in the exchange, therefore users would have both their voice and data moved onto the new 21CN in one foul swoop. I hear that some connections in Oban town centre are still on 20CN, even though 21CN and ADSL2+ is available, so BT appear not to be in such a great rush to migrate people off 20CN afterall.
2) There is a possibility that the 'point of handover' for the FTTC data to enter the 21CN WBC is not actually in the Ledaig exchange, rather at some other exchange larger exchange. In which case there will be no 21CN WBC at Ledaig, and no possibility of ADSL2+ MSANs at all. This source mentions several scenarios of small exchanges 'receiving' FTTC, but without the actual exchange being upgraded to 21CN WBC (e.g. Innerleithen exchange's FTTC terminates at Peebles Exchange, Falmouth exchange is the termination point for 8 smaller exchange's FTTCs in the vicinity)
3) [added 23/3/15]. BT are performing an 'SDH bypass' at the exchange, where traffic from each ADSLMax DSLAM will be ported over the 21CN backhaul, instead of leaving the exchange over the 20CN network. This effectively extends the life of the ADSLMax 8Mbps connections as the ADSLMax DSLAM data can be ported over the new 21CN backhaul.
IF the Ledaig exchange is to be upgraded to 21CN and IF the point-of-handover for the Benderloch FTTC is at the Ledaig exchange and IF existing DSL/20CN connections are also being migrated to 21CN and IF you live within 4km of the exchange, ONLY THEN will you receive an improved broadband speed over your existing connection.
Home -->[via copper]-->Exchange DSLAM -->[via BT 20CN network]-->BT Core node and the rest of the internet
Home -->[via copper] --> Exchange MSAN --> [via 21CN WBC] --> BT Core node and the rest of the internet
FTTC (via BT WBC)
Home -->[via copper] --> Street cabinet DSLAM -->[via fibre] --> Exchange Layer 2 Switch -->[via 21CN WBC]-->BT core node and the rest of the internet