Thanks for writing in!
1.Please follow our X to Pro update guide here first (https://theme.co/apex/forum/t/x-to-pro-conversion/104). You site should be remain as same even after you have updated Pro. However, since we don’t know what type of customizations that you have done to your site or the other 3rd party plugins that you have used, I would strongly recommend to take a full backup of your site before upgrading.
2.You can assign custom headers and footers to your website after you have upgraded to Pro. Please check our Header and Footer builder introduction here (https://theme.co/apex/forum/t/pro-header-and-footer-builder-introduction/103).
The content area of your screenshot can be changed by head over to Pro -> Theme Options -> Integrity -> Blog Options.
3.All of your X theme options and settings will be saved in the database and it should be remained as it is, after you have updated your site. Just to be in the safe side, we always ask our customers to take a full backup. You can follow this guide on how to take your backup (https://theme.co/apex/forum/t/cornerstone-content-builder-migration/126).
4.You are correct on that. Pro has all the common functionalities in X theme with advanced features such as Header builder and Footer builder which is only available for Pro.
Hope that helps.