Question re: new ep and current mods

I am planning to get Ambitions when it is released. I am just wondering how a new ep affects cc? I am new to sims 3 and am used to sims 2 which I had nearly all of the eps and played for almost a year before ever using cc so I never had a lot of problems with outdated cc. I have quite a few mods although not nearly like I did for sims 2. I have clothing, hair and some mods. I planned on removing all of my cc and then installing the new ep. I am mostly concerned about my hair. I have had the most trouble with hair (especially peggy hair). I had found a post saying that I needed to use a tool to make my hairs ep ready. I did that for the problems I was having since I have Hel and WA and now I have a bunch of hairs that I can't get out of my cc that are either bald or look like black burnt scalps. I just don't want to do that again with the new ep.

I am also kind of confused as to how cc works. I know how and where to install it depending on what kind it is. I just don't understand what the difference between the package files and sims3pack are. Why are some things I download package files and others are sims3packs?