Computicket - Paying for forced ticket changes

I booked 2 tickets for the opening night of Rock of Ages at Monte Casino. Then got an sms from Computicket saying that they have canceled the performance, and need to move it to another day. But I understood that these things do happen, so called them to change tickets but they insisted I needed to go into a store to change it. So this is already an inconvenience, then when I got to the store I asked for a specific date and they said if I want that date I need to pay more. So you send an sms saying I need to change the date but changing the date will cost money!!! Are you kidding me, this is pathetic customer service and completely unacceptable!