1.) Not possible, sorry. It's not possible to upgrade a CX ticket with any mileage currency other than Asia Miles. Even then only cash fares are eligible (no reward tickets), only some cash fares at that (Y,B,H,K,M), and upgrades are one-class only, ie Y to PEY. CX cash tickets can also be "upgraded", fwiw, but it's generally only via paying the fare difference...which is really just like buying the premium class outright. Regardless this isn't applicable to your situation.

2.) If I misread your question, and instead it's about how to re-ticket using Avios in a higher class (ie, cancelling your Y class booking and burning Avios for a pure PEY, J or F award instead), you will need to ask that in the BA forum.