Naïve question : less harmful way to clone a module ?

While not wanting to hack Drupal's Core (OK, I do not want to kill kittens), I would like to clone one of them, typically "forum", in order to understand it better by tampering with it. The first step seems to copy the whole modules/forum directory to sites/all/modules/forum and then to modify what has to be modified... but, question, what must/must not/may be modified ? What I wonder is :

1. Is it OK if the directory name to stay the same ? In other words will sites/all/modules/forum supersede modules/forum ? Should I at least deactivate the "forum" part of the modules, or will the activation automatically be transferred to the more specific copy instead ?

2. To avoid name conflicts, should I instead change every occurrence of "forum" in every file content, file, directory to something different, like "forOOm" ? If yes, is there a tool existing somewhere that would ease the process ?

I also understand that I should change the forum.info (or fOOrum.info, in the second case) package name so it will not be "core" anymore. But the question I cannot figure out is : what is useful, what is useless ?

Any link on a solution would be welcome. If anybody has one. thanks in advance !