Jarvis,
If you transfer to Cabin Crew from another department, your "Cabin Crew" seniority will start fom the day of your training course (i.e. 1 AUG 2007), you will be allocated a ranking within your group based on your exam performances and also the BA service style standards (so no.1 would be most senior in your group, etc).
You will of course retain your DOJ for staff travel and pension benefits, but all Cabin Crew issues, (i.e. fleet transfers, eliligibility for promotion, work positions onboard, etc will be based on your Cabin Crew course date).
Hope this helps.