Vodafone — Post paid plan change to prepaid due to no plan provided in corporate sim

Dear Sir,
I was using my mobile no. [protected] from 2007 year in same idea vodafone network, i am your valuable and loyal customer. last time u had change my plan automatically without my information from 149 to 299 plan. and my mobile bill getting exceed. kindly give some solution for this and change to 149 plan.