I have decided to post one of my Harry Potter theories. Here goes:
OK, on JK Rowling's official site (www.jkrowling.com) she states there are clues hidden throughout. On the main center of the page there is a blue butterfly that flies around. Cho's name, in Chinese, means "butterfly." Interesting, right? Well, at night, this butterfly turns a greenish/brown hue. So, this is my theory. I think that between the fourth and fifth books the real Cho was switched out by the Death Eaters (or whoever--someone bad) by a metamorph (like Tonks). Why? Well, Harry showed an interest in Cho in the 4th book, but he didn't really get to know her. So whoever is behind this thought "She could get close to Harry, find out information, kill him [fill in the blank]." This would account for the increase in Cho attention in the 5th book. But Harry's a little unpredictable and Cho's constant crying, instead of drawing him closer, drives him away. Now, why Cho? Well, Harry was interested and Cho knew it. If one of the Gryffindors or another closer friend were replaced, Harry (or probably Hermione) might have figured it out.
Well. This brings us to the DA. If Cho were a bad witch she would want the DA to be stopped. Enter Marietta Edgecomb, who is dragged into it by Cho. Bad Cho would realize that Marietta would eventually crack and tell a professor. Thus, the DA (she thinks) would be stopped and she wouldn't get fingered for it. This would account for her taking Marietta's betrayal rather lightly.
Also, JKR is good at making characters likeable, but I have encountered few people who actually like Cho. This makes me think that perhaps we aren't supposed to like Cho in the end.
Back to the butterfly, you see the possible significance of the whole "change in color" thing. First a Ravenclaw (blue is one of their colors) and then a Slytherin at heart? I think part of the point for having Tonks was to alert readers to the possibility of other metamorphs. She changes her appearance so often and there usually isn't a real point to it.
Well, that's my theory. I'm fairly confident, but who knows, really. It's fun to make theories though.