I upcycled/modified a shirt the other day and thought I'd share!
It started off as a loose fitting white linen/rayon Gap shirt from Goodwill. Linen/rayon is my faaaaaaavorite fabric blend. I love sewing with it, I love wearing it. It has the softness and breathability of linen but the rayon keeps it from wrinkling too much. But I don't like stark white clothing, so I started off by dying it with some
dark green Rit dye.
(As a side note, I know Rit dye is not the highest quality dye. However, I wash my clothes in cold water and hang dry so while YMMV, I haven't had any issues with fading or color leeching.)
I also took off the "Gap" label from the side (see photo below) because visible logos peeve me.
I sewed the button band shut because I have tig ole biddies and they cause gaping. I don't wear super fitted clothing (sensory hell) and I also don't typically wear buttoned shirts open (like, over t-shirts or tank tops) so this is a perfectly acceptable solution for me. I actually really struggled with the best way to do it on this shirt. I usually whip stitch the innermost edge of the band down from the inside. I like the more natural look this gives, since the button band can kind of still move around. I find sewing the outermost edge of the band really makes it look obvious that you've done something unusual to the shirt. But the weave of the shirt is so loose that even teeny little whip stitches were visible. I hemmed and hawed for a few days before deciding to stitch over the top stitching two or three times every inch. And I'm really happy with how it turned out!

You can see that every few stitches on the left hand column is actually a hand stitch done over the machine stitch. It's not noticeable in person. Here's what it looks like from the back.
I did every few stitches "backwards" (like
a backstitch) so it wasn't just a line of
running stitches that could pucker or shift around unevenly.