I just found out the this winter the Matterhorn Alpine Crossing and that if I have the half fare travel card it will cost me half the ticket. Having said that, I already bought my tickets under day pass from Zurich to Zermatt. I did not buy the Half Fare because it did not make sense only for the trip from Zurich to Zermatt and back, but if I include the Matterhorn Alpine Crossing is much cheaper. The specific question is. Can I change or upgrade by buying the half fare now