Following the instructions, I basted in several guideline stitches of yarn -- one in the exact middle, and two five rows away on either side. Then I machine stitched one row away from the middle on each side and then two rows away from the middle on each side. So, those cut stitches have nowhere to go. Then, with wild abandon, I took out my trusty shears and cut my sweater right up the middle on the basted yarn guideline.
So far, the sweater hasn't completely self-destructed. In fact, it looks pretty good and, better -- now that I can try it on properly -- I think it might actually fit.
Now I need to pick up umpteen million stitches along the edges and knit the dreaded button bands. This is Stephanie Pearl-McPhee's least favorite thing. I guess we'll find out how like it.