Giga 100

My giga 100 is coming to an end on 22 june (tomorrow). I went into the cellc branch at cresta to purchase another giga 100 onto my existing number only to be told I cannot add it to my existing numbe as the sim cards come prepaid and the only way to take out this deal is with a new sim. I find this higly inconvenient as I need to keep my number. As a customer of this deal I cannot be expected to change my cellphone number every 12 months.