I am on Top-Up, phoned them.. and there was a long winded, no real clarifying explanation to me, why I am not able to chose a different billing system.
So, now I'm being "punished" for being on a different system or not a new customer, on the 141/147 dial options it says that you are able to change your tariff plan, but there's no option ether on the self-help phone option or through an operator.