Showing the original pattern piece and the new fuller longer pattern overlaid so you can see the underarm and side seam should be near identical to the original. The length increases to adjust over the fuller bust. Try on and check that the waist line is straight on the body.
Busts can sit high or low, fullness at the sides or up high. The bra and shape will all make further adjustments necessary with some patterns but this is the basic principal to add fullness where you want it and avoid gaping, wrinkling and pulling elsewhere.