Thanks! It was super fun and easy (but it requires patience and a willingness to try on and pin and trim and sew repeatedly until the fit is right).
Okay, so for those who want the full, image-heavy tutorial, you can find it here: http://myhandmadehell.blogspot.com/2014/09/i-made-thing-monday-sweatshirt-to.html
Here is the shorter version of the tutorial:
First you'll want to remove the sleeves, the ribbing on the wrists, neckline, and bottom (if there is any), and cut a very super-straight line up the center-front:
The VERY easiest thing to do next is to take an existing cardigan that fits you well, lay it on top of the sweatshirt (with the sleeves tucked inside-out inside the cardigan), and cut around it to get the right width for the shoulders and the torso. I didn't have one, so I tried it on (inside out) and marked the top of my shoulder joint with tailor's chalk:
And pinned the sides so it was a nice slim fit, then cut away the sides and a curved line for the sleeve holes using my pins and chalk marks as a guide. I did this wrong - the bottom tapered too much which is why the cardigan doesn't close all the way down. I highly recommend leaving the sides straight - just pin right under the armpit and then draw a straight line down to the bottom.
Then, after trying on and adjusting the fit as needed, trim the width of the sleeve you cut away earlier so the larger end fits the new sleeve opening better (do this by laying the sleeve next to the sleeve opening and drawing a line with tailor's chalk before cutting it):
Sew up the sleeve and attach using your preferred set-in-sleeve attachment methodology. My picture here is awful but there are a zillion much better tutorials to be found online doing the same thing:
Then I used a fancy stitch on my sewing machine to hem the front center opening of the cardigan:
And added lace to the neckline and sleeves for that extra "something". Once that was all secure, I struck all the poses!
I would have liked to be able to have buttons all the way down, but my sizing mistake made that impossible. So instead, I keep the top closed with a vintage brooch. I feel all kinds of Donna Reed in this thing!