Need to customize sidebar.php

Hello, I have a few questions:

1: How do I edit the various default sidebar items such as the category listings – I want to edit the actual HTML on each anchor so that there is a span within each (for styling purposes) but I cannot figure out where to change this. I know it’s got something to do with wp_list_categories(). So, for example, I want to have the list as it is but with <a><span>text</span></a> instead of <a>text</a>

2: If I want to modify a bit of PHP in “post-template” or “category-template” under /includes then surely my changes will get wiped out when I upgrade to a newer version of WordPress, right?

3: I have five pages. On each page I want to specify what sidebar items show. I don’t have to have this functionality within admin panel – I don’t mind doing it in sidebar.php … something like:

My ideal sidebar.php –>

----
if(page=="gallery"){
   showPhotosWidget();
}
if(page=="contact"){
   showContactInfo();
}
if(page=="blog") {
   showCategoryListing();
   showRecentPosts();
   showMyBookmarks();
}
----

I know those are not the functions or watever but hopefully you see what I mean…?

ANY help is really appreciated! ??