Assuming this is a QR booking, they would almost certainly regard that kind of amendment as requiring there to be availability for making a new redemption booking from revised origin to final destination. That is usually easier said than done on the same flights without cancelling the current ticket and hoping the seats go back into reward inventory, which is not guaranteed.

I'm not sure they would offer a redemption from NCL via DOH for that route anyway (I can only see availability all on BA metal), although they will sell it as a cash ticket.

So I would suggest leaving the current ticket well alone, and booking a car transfer T5 to T4 if the luggage will be a problem on the tube or bus.