Was the abutment drilled in too far, or has skin grown up over the post? The abutment is supposed to be below the skin level and can be left in place and closed up if the Baha isn't right for you. Overgrown skin is a common problem, and moving to a longer post is a simple procedure that can be done in the office. It shouldn't require surgery.
If the abutment was placed too far into the bone (or at a poor angle), I do believe they would have to move a specified distance from the first abutment and place a new one during another surgery. But I think that would be extremely rare.