Fortunately, the WordPress CMS platform is pretty flexible. You should easily be able to maintain your current site structure within the WordPress platform.
The following links should contain all the information you need to know to configure your WordPress blog to emulate your current website structure.
Pages « WordPress Codex – You can use pages, instead of posts to emulate your current website structure.
Creating a Static Front Page « WordPress Codex – This will explain how to set up a static homepage.
Using Permalinks « WordPress Codex – This will explain how you can use the permalink structure to emulate your current link structure.
WordPress › All in One SEO Pack « WordPress Plugins – This plugin will give you the flexibility to make sure your new page titles and descriptions match your current web pages.
http://codex.wordpress.org/Appearance_Menus_Screen – This explains how you can customize the navigation menu to emulate your current website navigation structure.
http://codex.wordpress.org/Editing_F…_Plugin_Editor – This will explain how you can modify any of the theme pages necessary to emulate your current page elements.
I would recommend that you set up a second site if only temporarily, to get all of your configuration done. Then back up everything and restore it onto your current location. Do a backup of your old site so that you can reference it if you discover you missed anything after the transfer.
If you are switching hosts then you can just set up the new website on the new host account and once everything is configured to your satisfaction, update the DNS record to point at the new website hosting address.