they normally only update the checker with an approximate date if it is known as all dates are quarterly place holders accurate activation dates are then posted al you can do is wait I am afraid
In my own case it vanished from the checker and then appeard again only when the cabinet went live it is nothing to worry about
Did Openreach actually tell you that it was going to be upgraded by March 2013 (give or take for delays etc.), or that this was a review date after which further work or investigations would take place?
In my case the FTTC cab went in 11 weeks and 3 days before it went live and the date given by the BT checker kept moving about.