(Updated section is in bold)
The common factor that's changed here is the age of the two of them. As of now they are aged 15/16 (I haven't decided yet a specific age)
Other than that they're obviously physically more developed, slightly taller and their hair is spikier. Asuna's hair is also a bit longer, something which it seems nay have been influenced by a recent poll.
I wanted to change Gesen's complexion and give him a bit of a tan, but when I did this fans commented that I made him too dark, so I decided to leave him how he is.
So, how exactly are these redesigns going to be integrated into the manga?
From chapter 6 onwards, several characters including Gesen and Asuna will have slight redesigns. I would have liked to have been able to redo former pages for consistency, but i'm going to write it off as development in style. After all, Chapter 2 looks quite differently to the rest of the comic, right?