If you've got a Reseller account which you'd like to downgrade to a User account, the process is fairly simple.
We'll assume the Reseller's name is fred, and fred will soon be a User.
We'll also assume that fred has no Users below his account.
1) Convert fred's account type to be a User. Edit
/usr/local/directadmin/data/users/fred/user.conf
and change the line:
usertype=reseller
to become:
usertype=user
2) Update the lists. Edit:
/usr/local/directadmin/data/admin/reseller.list
and remove fred from it.
3) Edit:
/usr/local/directadmin/data/users/admin/users.list
and add fred to it.
4) Give fred the bad news that his account has been downgraded.
We don't need to delete the Reseller-related files, since they won't hurt anything. They're also handy in the event that you want to upgrade him later.