I frogged the tops of the sleeve caps and seamed them in with a bit of a seam allowance on the body.
Then I seamed the sides in several inches, tapering from the bottom to the armscye. I caught the edges of the new seam allowances (for want of a better word) all along the sides.
The alterations are invisible from the front. That’s the tip of the crochet hook showing through so you can see it’s the same location.
The new and improved sweater.