I've just updated an Exia that I assembled about a year ago (when I started making Gunpla).
I made the shoulders of the upper body smaller, set the face a little further back, and added a ball-motion mechanism to the abdomen.
For the lower body, I changed the leg connection from BJ to the axis connection that is common in HG, and added a roll axis to the thighs.
It was my first time cutting and pasting a Gunpla model to this extent, so I was a bit nervous at first lol, but I'm happy with the result.

A reasonable HG Gundam Exia modification | ウォン's Gunpla works