Converting your existing website into a Content Management System (CMS) is one of the most beneficial upgrades you can perform on an existing site. A CMS can simplify every day tasks that are often undone because they require a request to your site's webmaster or developer. A CMS allows you or your approved representative access to the site within a user interface that you can edit easily.
Upgrading to a CMS from a static website often does not require any site re-design, although if your site was built before 2008, chances are that much of the code is depreciated for modern browsers and will not perform well for today's screens and media devices. Depending on your site's structure and code, an upgrade to a responsive design might be suggested.
So you have an existing static HTML site that you would like to convert to a flexible and mobile site.
Or, you would like to keep your existing site, but provide an easier way to edit the main content.
Either way, my suggestion would be to convert your existing static HTML site to MODX.
Site conversion to MODX is the preferable method due to MODX's incredible flexibility. Any HTML page can be integrated into MODX, including all PHP and mySQL functions.
Anything your static site can do, MODX can do it with ease plus you get the additional benefit of the MODX Content Manager. Allowing you to quickly modify your own content.
Please see Content Management for more information.