Hi, I am considering the following situation and wondering if anyone has experience with something similar:

I will be booking international flights with AA and using my avios points. If I was to book Business in that way, and First is available on selected flights, can I then upgrade further using avios once I have made the booking? I appreciate it depends on availability, but let's say that there is. Basically, can an avios-purchased flight be further upgraded using more avios?

Thanks for any insights you may have