Apologies for any confusion.
X and Pro are similar in a number of areas. Once converted to Pro, the original X license cannot be used because it is replaced by the Pro license. Regarding Cornerstone, it is not required in Pro because it is baked into the Pro theme. In other words, you'll have the same page building functionality in Pro but you do not need a separate plugin to provide it.
If you need any specific help regarding the set up or use of Pro (or X for that matter) we are always on hand to help. You can also check out our knowledge base articles here: https://theme.co/apex/forum/t/knowledge-base-overview/287
If after checking out the above you're certain that you would rather use X instead of Pro, let me know and I will convert your license back to X (and refund the difference paid).