How can I convert a V-neck shirt to a square neck?
I recommend that you ignore the depth of the V in your v-neck shirt and cut the square where you want it to be (allowing for seams, obviously). Then, using the cut-out material, you can use it as a modesty panel to fill in the space where the tip of the V is to make a full square neck.

What you do with this notch is up to you. You can try to make it blend in by stitching it flat or you can use some of your embellishments to make it an accent to the center of the square.

The final result would be something similar to the neckline here:



Though, I'm guessing the V left over is going to be shallower than this one.

The obvious benefit here is that you don't have to muck with how the shirt fits. It's not been recut. All you've changed is the neckline.