Hello,
I'd like to share my experience with the voucher.
I called BA call center here in Italy and asked about refund options for unused Avios + cash remaining on a voucher after a rebooking has been made. They confirmed that Avios will be credited back to the BAEC account but cash will remain on another voucher unless amount is lower than 20GBP. In such case amount will be credited back to the card used for payment.