Same issue; the main account email ID was never used for primary daily email; three aliases (myself, wife, & daughter) are the addresses we would want to transition. I tried transitioning my daily email address (an alias) and yahoo defaulted to my main account E-MAIL. It knows the alias is associated with that primary email address it defaulted to. I'm getting email at yahoo for the address I don't need.
I can go to Cox webmail for my alias and it still works; said once you are transitioned it will automatically go to yahoo.
I contacted Yahoo help and was directed to creating temporary email addresses (their equivalent to aliases) but that option is no available unless you have the paid Yahoo Mail Plus.
Cox has done a poor job dumping their long time customers utilizing their E-MAIL for their family as designed when we started with cox.
Bob