my %new_client_hash;
foreach my $old_key (keys %clients) {

   my $new_key = $clients{$old_key}{new_sys_id};
   delete $clients{$old_key}{new_sys_id};

   $new_client_hash{$new_key} = $clients{$old_key};
   $new_client_hash{$old_key} = \$new_client_hash{$new_key};
}