If you are willing to pay cash then the underlying ticket should not matter as much. CX does not have a published policy on airport cash upgrades though. Only limited anecdotal data points are occasionally reported here. Don't expect it to be cheap. It could just be calculating the difference between your original ticket and the business class fare. Airport agents may also not have enough time to process if you arrive close to check-in cut-off. You may as well try to buy a Business class ticket straightaway or change your existing ticket to a Business one. Availability may be better.