@TheDude, thank you for your help.
@TheDude is correct. Please also check this article: https://theme.co/blog/say-hello-to-x-five-and-pro-one/
Regarding customization, as long you have used the child theme, it is easy to transfer. Feel free to download PRO child theme from your dashboard and then move your customize templates. Also note, that there's some changes on footer and header file structures so you might need to update that too, but that is only applicable if you customize those files.
Using PRO, we have the freedom to built our own header/footer the way we want it using the header/footer builder. In X, the design and structure is already set per stack, we just need to tick on and off some settings.
Please check videos first on how Pro works from knowledge base here.