Yes, you can do that but we won't advise that, once you have finish building your site in one theme, you should not switch it to other themes (not only for X and PRO but for other themes as well), unless you're planning a rebuild.
That means if you're planning to switch in PRO at some point then please consider building the site with PRO from the start, X and PRO are basically the same. The only difference is PRO has the Header and Footer builder, which you have the option if you want to use it or not.
You can download PRO from the APEX dashboard
Hope this shed some lights,