I may be getting mixed up between two aspects of this, but it seems that quite often on WBC-enablement of an exchange users can be moved from the old DSLAM onto an MSAN simply as an technical move. No service upgrade. Then (if the ISP requests?) a bulk system upgrade is carried out on a phased basis to ADSL2+.
Didn't know they ran the old service on the new kit, customers generally seem to stay on old kit unless moved by ISP regrading to WBC product - are BT Retail selling two different WBC speeds ?
Certainly the upgrades to ADSL2+ are done on this phased basis. My possible confusion that I referred to is whether the physical connection move and the ADSL2+ upgrades are simultaneous. I don't think it normally is.
It is possible for a user to be upgraded before the bulk job by the ISP paying £11 for that connection to be sooner.
Then we get oddities like Plusnet who have a default 448kbps upstream cap on ADSL2+ connections unless the customer requests it be uncapped. At which point they are warned this could destabilise the line. .