This is a useful link for how you might use them for upgrades:
https://www.qatarairways.com/en/Priv...o%20fill%20out.
Check the T&Cs at the bottom. Some fare buckets are excluded.
Generally speaking, there would need to be availability to book a standard price redemption in the higher cabin in order to upgrade in advance with points. Or upgrades at the airport would be subject to space and catering available.
You can leave your avios at BA until you need them, transfers are instant in my experience, or the linked accounts feature should draw from both if you don’t have enough at QR. But you may need a minimum number in your QR account first in order to search for award space.