A friend upgraded from an iPhone 5 to an 8. They just treated the new phone as a new device than than use an iCloud backup.

Anyway they cannot setup Yahoo mail on the new phone because they have lost the password and the recovery account is also not accessible. But the iPhone 5 still has the mail working.

I have emailed Yahoo support to ask for some help but I wonder if people more experienced than I am in IOS might have a way which just uses the fact that the old phone still has access. 

I did play around with Keychain or whatever but that didn't help. But then not being an IOS person I really didn't know what I was doing :-(

I am not sure they are even doing the equivalent of saving passwords in Chrome so I could take a look since they only ever use the phone to access emails. 

Thanks