Welcome to the community!
Switching from a pre-paid service to a post-paid monthly plan might be an option. However, it would not be possible to migrate the services online. As far as I am aware, the migration can only be done by contacting customer service to have them switch your plan types (see here). In addition, they can also be contacted via Live Chat, Facebook, or Twitter. Those methods can be accessed via the contact page posted above.
You should note there would be a migration fee required which would be applied to your next post-paid bill. You should also note that if you chose to switch back to pre-paid services, you would likely incur an additional migration fee.
In addition, you should further note that a credit check would likely be required prior to switching from a pre-paid to a post-paid plan (see here).
Hope this helps 😀