I can't upgrade a couple of Cathay sectors online (Avios or Cash) to a multiple journey ticket to Australia... (I'm doing LHR to HKG with BA and onwards with CX under the same booking). The whole booking is in WTP/PE and I wish to upgrade to business, depending on availability. I know I can't use Avios on CX legs and there's no reward availability yet on any sectors (I've been advised to wait until nearer the time). I can't do anything online as it says to contact the call centre - probably as it's a relatively complex booking.

Are the BA Call Centre able to upgrade the CX legs or should I call CX? The last time I called the BA call centre I was on hold for 15 minutes so trying my luck here first.