T-shirt Necklines

I have a bunch of women's t-shirts (the form-fitting type) that have high
necklines. I would like to lower the necklines, if possible - preferably
scoop and not V. I'm not sure how to do this (without stretching, puckering,
etc.).
Is binding necessary on these mostly 100% cotton T's, or can i turn under and
stitch? What type of stitch do i use?

Any advice would be appreciated. Used to sew a lot years ago, but lately
it's limited to a mending or altering.

Thank you.