[SEOvB]

Before you upgrade, contact your host and ask them to backup all of your forum data for you, including the database. Then, make a backup yourself and store it on your server. Then, download a backup. I've upgraded probably close to a hundred forums, and I've learned that you should always always back up your important data, and then make redundancy backups if you're nervous.

But to answer your question, no you won't lose anything by replacing those files. Those are the files that control how vBulletin work, but they have nothing to do with your stored data (posts, users, etc.). The only thing of consequence you might overwrite would be a custom favicon.

Still, if you're this nervous about it, why don't you hire professionals to do it for you?

-Nick, SEOvB Developer