JQuery :: Make A Two Part Navigation Menu (main/sub Category)

My goal right now is to make a two-part navigation bar where you don't have to reload the whole site, when you click on a link. The navigation bar consists of two <ul> lists with <li> float:left and <a> display:block, so I basically have two lines. It's probably easier to understand if you see it for yourself: jsfiddle (had to change the css style, because I use .less but it works)

When you load the page it's just like that, you have your main category (A,B,C,D,...) and the sub-category (A1,A2,A3,...). What I want: If you click on B, the sub category changes to B1,B2,.... If you click on a sub category the site loads the requested page (with ajax = best solution?!)

My thoughts

At first I thought about just changing the name and href attribute of the second list, but the main categories have different amounts of sub categories, so I would need to add or remove <li>'s. Then I thought about using .load() but I somehow screwed up (of course) and the loaded <ul> doesn't have any css style (wrong order?) and I can't see anything. (with a loaded test.html site containing only the word "test" it worked) Is ajax "overkill" as a solution?

CodeIgniter and the URL

I use CodeIgniter (a PHP framework) for the website and URLs like this: www.example.com/maincategory/subcategory (e.g. example.com/A/A3) That's no problem at all, if I don't use any javascript and reload the whole site every time. But is it possible to change the URL with jQuery when the user clicks through the navigation bar? You click on B, the URL changes to example.com/B all the subcategories change to B1,B2,...When you click on B2 the URL changes to example.com/B/B2 and the site loads the requested page (I guess with ajax, so the navigation stays the same?!)

Minor problem: when you load the site, click on B, then B3 for example, you would get the url example.com/B/B3. If you now save this link, close the site and reopen it with the link, CodeIgniter would load the right site and everything, but the navigation would show A1,A2,.. in the sub category.

Hover vs click

Almost every page where I saw this kind of menu they used hover to change the sub category. But I thought click would be better, so you don't have to avoid touching B,C,D,.. if you want to go from A to A9 for example. Only downside is that you have to click before you see the sub categories.

Summary

A two part navigation menu you can use without reloading, changing URLs (when possible) and probably ajax to load the actual content.