You can update your MFA options at any time. You will need to do this if you change your mobile phone, or want to use the Microsoft Authenticator or Google Authenticator app.
Before disposing of your old phone, you will need to use it to authenticate access to your Microsoft security page.
To update your MFA settings, visit: Microsoft security page (https://mysignins.microsoft.com/).
There will be no charge for receiving text codes or if you are using the App connected to WIFI, however, the App may use a small amount of your mobile data if you are not connected to WIFI. If you want to avoid this please ensure your App setting is altered not to allow use of mobile data or when you are prompted to authenticate on your computer – select choose another option and select receive text message or enter verification code from the App.