So you essentially want to take it in 3/4″ on each side. I wouldn’t consider the change to be an extreme one. First, draw and outline of your total pattern. Then pivot inwards 3/4″ and redraw sleeve cap to center of cap. Replace pattern to original outline and repeat process on opposite side. Side the pattern horizontally over to one new side, trace the sleeve side seam, do the same for the other side and draw a line at the bottom of the sleeve hemline. I am concerned that your sleeve may be too small to fit in the armhole so make sure to do a muslin to test this. You may have to take in the side seam a bit at the bustline to make the new armhole fit again.