Kinda tricky if there isn’t a sideseam… If it were me, I’d use a crochet hook, attach a slip stitch, at the midway point, on the seam edge, and tighten it up by putting the hook into a stitch, pick up loop from live yarn, pull through, move over to next knit stitch and repeat, snugging it up as you go. Repeat until you get to the midway point on the other side. I’m not sure but if this doesn’t work, you can always pull/unravel the crocheted chain looping.
How much ease do you need to take out? I just worry that the sleeve will cling to the arm and the body will look like it’s too big. If it’s not more than 1 to 1.5" too big, the shoulder should take care of the problem, because you can take up 3/4" without it affecting the neck or yoke areas too much And, God forbid, you might have to remake the sleeves and and change the instructions.