It keeps everything. To answer your question, yes you can edit. I've drastically changed dialogue in early pages that I found half assed or just bad. When I first started I was trying to stick to a schedule and so I had to rush a lot of things. In retrospect a lot of the weakest points of my comic were easy to fix, I just had to gain the guts to cut significant portions of dialogue. I feel a lot better about the early pages since doing so, so I changed the image files. My comic isn't very big, so I figure no one will notice/care anyway, so I have that freedom. (Now if it had gotten popular, I'm not sure I'd be able to make edits like that.)
The part that gives me pause though, is when people make comments aout the dialogue they liked, but I ended up cutting. I'm sure they understand why I make these edits though...