You could do:
Code:
cp -R /home/username/mail/mydomain.com/abc/{cur,new}/ /home/username/mail/mydomain.com/xyz/
Above please replace username with the cPanel username, mydomain.com with the domain name, abc with the old email user's name, and xyz with the new email user's name. This will copy the cur and new folders on the abc account to that xyz account. Since maildir formats have each email individually named, this shouldn't overwrite any emails on the new account.
Of note, you might want to backup the mail folder before doing the above:
Code:
cp -R /home/username/mail /home/username/mail.bak
Again, please replace username with the cPanel username for the account.