I’m about the same. Last switch was yahoo to Gmail and now I’m in the process of switching from Gmail to Proton Mail. The most difficult part was the mental switch - which email you start giving out when you sign up. Second was migrating services. I made it a point for a while each time I logged into something knew I would switch the primary email. Its been a long process!
For everyone’s sake it might be best to refuse to support the application because of the security risks with such an outdated program.