There are quite a few internal changes between the Pint and Pint X, not just the battery pack. As such, you cannot upgrade your Pint to a Pint X.
That said, keep in mind that the Pint is designed and built to last and has many of the features of a Pint X. If the extended range offered by Pint X is something that you would like, consider picking one up and keeping your Pint to ride with a friend!