I booked return flights from LHR-SIN using my Barclays voucher to upgrade from WTP->CW. I would potentially like to change my return flight to another date where only there is only Avios availability in WTP. Does anyone know if it's possible to do this and get a refund of the Avios difference (as if I'd booked WT on the return leg and the voucher had upgraded me to WTP)? Would it also be possible to change the starting point of the return leg *and* the cabin if possible? E.g. to switch the return to a HKG-LHR in WTP.

Many thanks in advance for any advice.