Depending on who the carrier is, you should be able to call there and add the new number to the existing account and take the old one off. Some will make you create a whole new account and have the remaining balance returned; they won't transfer the money.
https://www.inmateaid.com/ask-the-inmate/how-can-i-get-the-remaining-of-my-phone-account-added-to-a-new-number#answer
Accepted Answer Date Created: September 07,2016