MFA Change from app password to ms authenticator app

Question

Hi,

We have a customer that is using MS App password, so they told me when they first configured outlook they inserted the app password instead of their MS Office 365 account password. What we want now is to turn on conditional access, and enforce MFA with authenticator app and block legacy authentication.

How will this impact the user if they are

1. Logged into Outlook - Will this just fail, or will be forwarded to Microsoft page to register for authenticator app ?
2. Start outlook - Will this just fail, or will be forwarded to Microsoft page to register for authenticator app ?
3. Using iphone build in mail client - Will this just fail, or will be forwarded to Microsoft page to register for authenticator app ?
4. Using iphone with outlook mail client - Will this just fail, or will be forwarded to Microsoft page to register for authenticator app ?

I am wondering what would be the best steps to accomplish this.

I have never worked with app password, so I am not familiar with how it works, and if we disable it how will that impact the user.

Thanks for any reply

/R
Andy