Date and time changes can be made to tickets booked with a companion voucher. I have done so several times. (Name changes are not allowed.)

The fee for cancelling the companion tickets, if done over the phone, is the same amount for changing the date/time ($70), so I don't see the point of doing a cancel/re-book. Also, you can do date/time changes online for free, no charge. However, after a few changes, that option online is greyed out, and you have to use the phone and pay the fee.

The fee for online cancellation of a reward flight (and redeposit of the miles) is $50 for each ticket.

Here are the fees:
https://www.britishairways.com/trave...l/public/en_us