worst service by Hugo from Ticketmaster

I booked Liefde by Die Dam tickets through Ticketmaster as they are the only ones selling the tickets. I took 11 adult tickets and one child ticket. My son won't be able to go due to sport trip coming up, so my daughter said she will take her friend we should just get the ticket upgraded to a general admission ticket. Last year we had the same issue as my son also had to go on sport trip. Last year they were also fully sold out, but they upgraded my ticket to general admission and sended me a link to make the payment for the difference of the amount in the ticket prices. This year they boldly refuse to do this and say I have to purchase a new ticket (but this is concert is sold out) they I can have the other ticket refunded minus the fees. They do not provide me with a valid reason for not being able to upgrade my ticket to a general ticket. This is the worst service as you cannot even phone any one and the same guy Hugo from Ticketmaster just keep on saying they cannot assist in doing this no reason at all.