Just guessing ... but I imagine that their DOJ BA will be the date of acquisition of BMI which I think will be the start of the 2012/13 financial year this April, subject to regulatory blah blah.
I don't believe they will be flying in BA colours or SOPs on that day but will continue with their existing route structure and be slowly retrained / absorbed / rostered into mainline over a period of time 12 -18 months.
Purely a guess but I would put money on it.