yes u can do the upgrade at the check in counter with cash or miles. though there is no official rule but china airline is very happy if u can pay wtih miles or cash for upgrading. miles are sure to be more bargain. frankly, on some routes, espcially tpe to hkg, there are always a lof of passengers in the airport in waitinglist. They will offer seats to those who want to pay with cash or miles to let them go first with C class.