Redesigning Twi was an experience. Basically I just gave her a haircut because at first I made her hairstyle basically the same but with a bun, and I wasn't happy with that so I thought to myself "What have I not seen so far?" And the answer to that question was: An adult Alicorn with short hair. Now she's more reminiscent of her younger self yet still feeling older. I also wondered what happened to the mass amount of hair that Twilight used to have when I remembered that you could donate hair, so I decided that Twilight cut off most of her hair and donated it.
While I'm here, might as well share a headcanon I have.
I like to imagine that the key difference between Celestia and Twilight's rule is that Twilight attends much more events. She tends to try to blend in with the normal ponies still, as she wants to be seen less as a princess and more as a friend.