Using a larger armhole on a smaller size: After tracing the entire bodice except the armhole, I marked the end of the shoulder line and the top of the underarm seam (i.e., the shoulder point and the underarm point of the armhole for my size). Then, I shifted my traced pattern over so that the shoulder point of the size large armhole matched my marked shoulder point, and the underarm point of the size large armhole hit my side seam mark. It hits noticeably lower on my purple side seam line than where the cross-mark for the original underarm is. I then copied the larger-sized armhole onto my traced pattern. Through the entire process, the grainlines are kept parallel. (I had changed the angle of the shoulder seam previously, for forward shoulders.)
Shortening the sleeve while tracing. I traced the grainline and the top part of the sleeve down to the lengthen-shorten line, then shifted it down the required amount, keeping the grainline lined up, and traced the rest of the sleeve. Then I trued the small amount of the side seam that had become disjointed by the alteration.