In Hunter x Hunter at first Knov appearance was pretty bold and looked smart. But after knowing the fact that their enemies are stronger then he got scared. But being a Hunter why he undergoes such a huge change in appearance ? I mean how come his whole physical body changed ? They knew that the mission they are going into is really scary but what was the reason behind his complete change?
His appearance changed because he was so terrified. The idea was that he was so scared of the beings they were exposed to that his hair went white and he rapidly lost weight.
Stress can cause rapid weight and hair loss. I don't know if hair turning white from fear is real but it is a common belief (see crash test dummy's MMM). That is what you are seeing exaggerated here. What you are supposed to note as remarkable is how he continues to aid the group and stick with it even though he is so debilitated. He is more scared but somehow the bravest GD person there.
The same thing happens to Welfin when he's in the precense of the King. For Nen users, Nen is an expression of their physical and mental condition so it makes sense that if their Nen gets disrupted, or if they're under a lot of stress, both the quality of their Nen and the appearence of their body can change. This also happened to Gon when he was so enraged and determined that he made an unconcious covenant with his Nen and his physical apperance changed. Knov is the same thing but more similar to Welfin than to Gon.