I e-mailed Jenny Glover on this issue last week and was told it is not possible, even offered to pay full price for a mate but was told the system can't accommodate this. Unless the poilicy has changed in the last few days.
with all the fixture changes I've maxed out on upgrades of my sons under 18 ticket and the club refuses to allow me to take a full paying guest in his place. Voucher is pointless. Club needs to be more flexible.