I am planning to book a Economy Essential multi-city ticket for HKG-SGN & SIN-HKG. Would I be able to use AM to upgrade the SIN-HKG leg [i.e., from Y to PE] only without upgrading the HKG-SGN leg [i.e., sticking with Y]? And if yes, is this something that might be problematic to do online on the CX website, or is it fairly straightforward?

Also, after I upgrade the SIN-HKG leg from Y to PE using Asia Miles, can I then use Manage Booking on the CX website to swap out my CX FFN and replace it with my BA FFN (I have BA Silver/OW Sapphire and CX Green, so I want to swap in my BA FFN once the Asia Miles upgrade is done) for BA mileage accrual and OW Sapphire benefits across the full multi-city HKG-SGN, SIN-HKG itineary?