There's never a downside to asking if this is something that would help you. Consultancies want you to be at your best, and no-one wants to waste their time with a candidate that just hasn't had time to prepare properly for one reason or another.
March is still far enough that BCG may be able to accomodate. Worst case, they tell you they can't and you come as initially scheduled. The people making the hiring decision are different from the ones doing the scheduling, so no harm done.