Let's see if it is theoretically possibly first, without retakes. I will assume you got the lowest possible mark for each grade (even though you didn't) so it may seem you have to score higher in A2 than you actually do.
B - 70%. To upgrade to an A you will need 90% in A2 (average is 80%) (coincidentally this actually gives you an A* since you got 90% in A2)
C - 60% To upgrade to a B you will need 80% in A2
D - 50% To upgrade to a B you will need 90% in A2
So yes, in that order, it is theoretically possible to get ABB from BCD. Note that for every increase in actual percentage per grade is equal to a decrease in the same percentage required for the target grade. I.e. If you C was 63%, you would only need (80-3=77) 77% in A2. If you got 79% as a B, you only need 81% at A2 to get an A.
Retakes may release the required percentage to hit the target grades, but may interfere with revision for A2.
I'm not that good with "inspiration" - I'm better with realism - You're gonna have to work your ass off to get those high A2 grades, and with those AS levels and the tougher standard of A2, I'd say get started as soon as you can. Don't leave it till Easter to revise, go over everything as you learn it. Test yourself weekly on last weeks content. I don't know what your other two subjects are but if they're facilitating subjects like biology then it's even more important. If you have any questions about UMS/percentages/grades or anything exam-related feel free to quote me.
Good luck.