resizing is an art.
in general the shoulders stay almost (but not exactly) the same size, and so does the neckline.
the increase are not proportional (you can't just take 2 stitches, here, 2 there, and 2 more somewhere else (and be minus 6 stitches and 1 inch (on front or back) and change a 38 inch bust sweater into a 36 inch bust sweater.
you can try.. and you might not be too unhappy with the fit..
most knits have some ease.. a 34 inch (actual bust measurement of body) might wear a 38 inch (actual measurement of bust of knitting) shirt..
why not take a sweater/knit top that fits (the way you like it) and see what it actually measures? you might be surprised.
if you still want to re size the sweater--be warned--its a lot of work to do it right