With version 6.19.1 , you now have the option to change your number once through the app!
First, go to your Textfree settings and tap Account info
Tap the three dots next to your Textfree number
On the next screen you will see an option for Change Textfree Number
Once you tap Ok, you will be able to enter your preferred area code and choose a new number
Since you only have the option to change your number once, it's important that you make sure to pick a number you really like!