Hi, there. I'm really sorry for the difficulties with your emails after the migration. You might want to go to Yahoo and generate an "app password", as this seems to correct this issue. here are instructions:
Open Yahoo page, and under Top Right-side point to You name letter next to e-mail icon and select Account - Overview
Select Security - top 3rd from the left side.
In the middle on the right side select: Generate and manage app passwords.
Under App name enter th name of the mail app you are using/having problems with (ie, Apple Mai for iPhone, Outlook, etc....l.
Yahoo will generate long password - Copy save it (as this will be the only time you will see it)
Paste this new password to Server Settings inside the mail client and make this your password.
I hope this helps.