How can I modify my iTunes login email address?

My email address on my iTunes log in is different to my Apple address so Im wondering how I can change it.

When I log into my Itunes account and click "edit" to change it I get a message "503 Service Temporarily Unavailable" (I have my music on a MacBook Air which is about 12 years old but still going strong).

I have just bought a new iPhone and want to download all my music on to it and I assume both addresses would have to be the same.All helpful advice would be much appreciated, thank you in advance

[Re-Titled by Moderator]