If you look through the forum you will see that there are apparently some problems with Yahoo.
I would advise that you don't change user names or passwords.
You could try the Yahoo server settings which may work for you. Be aware that these settings will stop working if/when you are migrated to BTMail.
IMAP
Incoming: imap.mail.yahoo.com Port 993 SSL
Outgoing: smtp.mail.yahoo.com Port 465 SSL
POP3
Incoming: pop.mail.yahoo.com Port 995 SSL
Outgoing: smtp.mail.yahoo.com Port 465 SSL