Hi there,
At the moment, you have one X license. As you purchased the license before the release of the Pro, you are eligible to convert your current license to Pro for free.
So by this, you will not get any additional licenses but you will convert your license from X to Pro. Then you can change the X theme to Pro on your WordPress installation per the instructions my colleague already gave the link to.
Please consider that this is a one time process and you can not get back to X as soon as you switch to Pro. By the way Pro is not an update of X and it has different features such as the Header/Footer builder and it lacks the Demo Content feature of the X. Instead it has another feature called Design Cloud which you can read more about here.
If you want to keep your X license and add another Pro license you will need to purchase a separate pro license by going to your Themeco Dashboard.
Thank you.