The first thing I would do is set up a testing environment using a virtual machine of some sort. VirtualBox or Virtual PC would be fine choices. That way you can start changing things without fear of breaking the production environment. No matter how much work this seems like it will be (with the database and web server and everything), in the end it will be worth it. One of the great benefits is you can copy the VM and give it to somebody else, if you find you need assistance.
