Would making several little changes to human characters make them more interesting?
Actually, I do believe making little changes can make any kind of character more interesting.
I have had an average joe character at one point, who wasn't very interesting. I made one little change, giving him narcolepsy, and now he has issues about spontaneously falling asleep. I would say it made him a little more interesting, or at least more fun to write about. I don't think the changes have to effect the plot. I would think it'd be enough if they simply just made a scene more interesting.
The thread got deleted, but you made a legitimate point that I wanted to respond to, so I thought I'd just explain myself here:
Your point about changing characters isn't really what I meant. When people talk about changing vampires, they're not referring to adding traits to an individual vampire character--they're talking about making arbitrary changes to the entire species. They want the vampires to be more unique. And uniqueness, in and of itself, isn't necessarily interesting. Hence my reference to Meyer. Making a change to an individual character in an attempt to add more conflict isn't what I was referring to, but I do agree that doing so can make a character more interesting.
Also, I misspoke when I said the changes must affect the plot. You're correct. Basically, if a change benefits the story in some way--however small--it's probably a good change. Thanks for pointing that out. c: