X and Pro are two different licenses. Once you convert X license to Pro, you cannot rollback or you will not be able to use both X and Pro licenses.
However converting X theme license to Pro will not be affected to your live site (you cannot update X theme, Cornerstone or you cannot install new extensions without a valid license).
Pro also comes with development and production licenses. So what you can do is to convert your current X license to Pro, then create a staging site and test it. Once you're done with testing, you can convert your live site to Pro as well.
Hope that's clear.