KARFA

Ok, to clarify things there are two different uses of avios you are talking about here:

Booking normal reward flights - so you can book reward flights on BA or other partners (AA/CX/MH etc.) and you can use avios to do this. You book through the ba reward search engine. You are normally better off booking two one ways rather than a return unless a) there is a 2-4-1 being used; or b) you are going to the US since exUS carrier surcharges are very high (in contrast exHKG carrier surcharges are zero so you may want to think about that if you want to book your return on CX). You can't upgrade reward flights with avios, however if higher cabin reward availability opens up later on you can change by paying the avios difference, change fee, and difference in taxes/fees/charges (usually none going from J to F). For reward bookings you earn no avios or TPs.

Booking cash fares and upgrading with avios - you can book cash fares and (subject to reward availability) can upgrade by one cabin by paying some avios and the difference in taxes/fees/charges. UuAs are only possible if it is a BA ticket and a BA flight from any selling classes apart from Q/O/G, or if it is an AA or IB flight booked through BA in selling classes J, C, D, W, Y, B – B class not available for upgrade on AA. As you can see UuA wouldn't be an option with CX flights. As with most cash long haul fares booking a return is usually far better value than booking two one ways. You earn avios and TPS based on the underlying cash fare and cabin not based on the upgraded cabin.

Hope that helps.