When moving to a new course such as doctoral course at JAIST, the user account can be modified based on the new student ID.

In this case, please follow the suitable steps from the below.

If you have previously used email alias, you can continue to use your old email address with your new user account. If you don't want to continue to use the old email alias, no need for this step. In addition, please note that your old email alias assigned to the old student ID (sXXXXXXX@jaist.ac.jp) can not be reassigned directly to the new user account.
You can alias here.

The data and files which have been used in the JAIST cloud desktop as well as the file storage can be moved to the new user account. 
For details, please refer to this.

The email messages in the mailbox of the old user account can be migrated in the new one's.
For details, please refer to this.

The email messages in the mailbox of the old user account can be migrated in a batch as a compressed file.

For details, please refer to this.