Create a duplicate database. Create a duplicate site as a subdomain on your hosting. Don't mess with the production site at all. Don't mess with the production database at all. Take your time, get stuff tested. Remember during your development time users are still logging to the production site, updating the data in the production database (depending of course on what your site does...)

When you are ready to deploy ask the following questions:
--Have I changed the structure of the database to add / delete tables, and fields? If not, its an easy thing... use the structure of the new site, with the original production database.

--If you have changed the database its a bit tricker, but overall not all that bad (phpmyadmin is your friend!!)

Make the change over when you think your customers won't be using the site... Errors do happen, stupid stuff will crash the site, your CSS files get hidden from the program at the worst possible moment, (don't ask me why I used that particular example, ouch!)

Good luck with it...

See

http://www.concrete5.org/documentation/how-tos/developers/organise-...