The email address used for your Asahi Net ID (ID Email Address, e.g., ab4t-ash@asahi-net.or.jp) cannot be changed.
However, Asahi Net Email Address (e.g., asahi.taro@asahinet.jp) and MyMail Address which allows you to use letters and numbers of your choice can be changed.

You can also have more than one MyMail Address. The first address is offered for free, but an additional fee will be charged from the second address.

For more details, please refer to the following page.
MyMail Service