The BS cars should remain in service until delivery of the final VL in the new order 3VL50, or TM 1317.
When is this scheduled to occur? Seems a bit of a waste fitting power doors to the S cars when they'll probably be withdrawn in a few years.
I believe the last of the current V/Lo order is due around 2012. That said, there may be further pax increases, which might necessitate SN7 to hang around even longer.
I can see more V/Los being ordered anyway (or similar intercity train)