The catalog configure.php doesn't have any reference to what the admin foldername is, and the admin configure.php cannot be read by zc_install until it knows where the admin folder is - chicken/egg. (And in v1.5.0, the admin configure file dynamically looks up its own location and sets the define accordingly, so it is not even hard-coded into the file at all.)
So there is really no way around renaming it back to admin for the upgrade.