A dedicated IP change requires 4 hours to resolve. Your site may appear to be down during that time. You can flush your computer's DNS or power cycle your router to speed this process up. You will NOT need to change your name servers.
Shared Web Hosting has only one cPanel. So you can have only one dedicated ip. If you choose the dedicated ip, all of your addon and subdomains will use the dedicated ip. Emails will still use the shared ip.