cant change the cellphone number when I upgrade

I phoned customer care and the upgrade centre and none of them can or want to help me.. I have a upgrade due soon and want to change the cellphone number but vodacom said I cant as I have a top up account.. then when I said that its going to cost me to go to another service provider they said I can go ahead but I cant change the number. I really dont want the same number as my ex is using the number and he will benifit every month and I will just fit the bill.. So guess I will be visiting another service provider then.