Here is what I would do, I have redone many pull over sweaters into cardigans, first spray starch the neck , then let it dry almost dry, then remove just a small amount of the shoulder seam so you can then rejoin the neck after you have cut it to where you want it, leave enough to turn under or put bias binding along the edge, use a stretch needle or your machine may skip stitches, you can buy the binding or make your own (your choice) move you needle if you machine has the function to the edge of the binding or seam I do this from the left side allowing the feed dogs a better track hold so it will feed smooth through the machine then sew the shoulder seam back, together match the neck with it. serge this seam or overcast by hand or machine. I hope this is a help to you. in you need more info give me a pm I will try to make it clearer.
Thanks for the details! I think I'll practice on an old tee to see what my results are.
Before cutting you could stay stitch (a straight line) around the neckline to stabilise the fabric before you cut it. I would remove all the current neck binding, then remove the hem at the bottom of the shirt and use it for the new neckline. Rehem the shirt using a twin needle to mimic the double stitching made by a coverstitch machine. Hope you find a solutions that works for you
If you want to bind it like most t shirts, you are looking to buy "rib knit fabric." Sold in some store in about a 4 inch band width, but not easy to find and you might have to settle for white, black or navy. Once you have the fabric you gently stretch and ease the neck band to the front side of shirt fabric, seam across the cut ends, then pull around to back/inside of shirt and stitch "in the ditch." Easier to show than explain. I learned this at Stretch and Sew in the 1970's when knits were "in" and sewed my kids' shirts.
Your explanation is perfect. I just haven't seen the rib knit fabric in years. I could just buy a cheap toddler's tshirt and make binding out of that.
Sorry I have no ideas at all. Its evening in the UK and my brain has ceased functioning. Hopefully there will be some Cutie(s) who are more alert than I am currently who can give you a sane answer.