Hi @Pitawat !
If you are looking for the smoothest way, the best thing you can do is communicate clearly with your staff.
You can also use the Microsoft Entra portal to change the default MFA method for users who HAVE setup the Authenticator app already but have SMS or other as the default: How to Change the Default MFA Method for Microsoft 365 Users
You should also take advantage of the Registration Campaign feature to encourage users to set up the Authenticator app. That being said, communicate to your staff first and set their expectations. Use screenshots!!
You can also use the Microsoft Entra portal to identify which users have the Authenticator setup and configured as default already:
- Login to Microsoft Entra.
- Expand Protection and select Authentication methods.
- Under Monitoring, select User registration details.
- Extract users who haven't configured MFA, haven't configured the Authenticator app or don't have the authenticator app as default.
You can avoid confusion by only communicating to those who need to be communicated too.