By converting to Pro, you will need to disable the Cornerstone as instead, you will have the Pro editor. The Pro editor UI is the same as the COrnerstone and whatever you have created using the Cornerstone will be intact and available in Pro.
For more information please read this article:
Please consider that you need to have a complete backup of your website before trying the conversion.
I also suggest that you check the articles under the Pro and Elements section to get familiar with the Pro related header/footer builders and elements: