Email migration if Yahoo account already exists?

But when I log in to yahoo using my (pre-exisiting) username@cox.net account, here's what I see, no cox email and no obvious choices to start viewing username@cox.net emails.

If you have a pre-existing yahoo email (not Cox.net,  but rather a Yahoo.com account) then it would be logging in with just a username that does not contain Cox.net at the end of it. It would still be completely separate and not contain any emails that are associated with your cox.net email account. If you can currently go to cox.com and can still log into and view your Cox emails there, that means your email address has not transitioned over to yahoo yet.

Okay, so it must be that some members of our household have been transferred but other's have not.  I'm still able to access (send and receive) emails from https://myemail.cox.net/ via my username@cox.net account, while others have already been moved.  Thanks, I'll try again next week.

Same issue for me. Although cox support told me my second cox address was ready to go