Are you thinking about installing Drupal but have an existing Web site to maintain during the changeover? This description of one conversion to Drupal will help you plan the change.
This conversion uses a local computer as the conversion and workstation. You install Apache, MySQL, PHP, and Drupal on the workstation. You configure the software then the current Web sites then merge the Web sites in to a new site.
You copy your current Web site to the workstation so that you can experiment with compatibility changes for your current site. You configure the new Drupal site on your workstation. You then merge the Drupal site and the existing site as a new Web site on your Workstation. When you are 100 percent happy with the new site, you copy the new site to your server.
I describe what I did. I cannot describe what you need to do because I do not know how your site is constructed. What is of use in my description is the overall process and, if you happen to have similar systems, most of the detail.
I have set up over 50 successful Web sites and helped hundreds of other Web sites become up to 1000 percent better. My experience means I do some things without remembering to write them down. I may no have written every detail in this page which means there may be some obvious steps missing.
I will include references to sites that help you with more detail or more up to date information on specific steps. Send me your questions and feedback so that I can improve this information for the next person who wants to convert a Web site.