Vodacom Existing Data Package Ripoff

I have a data contract for 10g anytime and 10g night (which us useless to me anyway) and I pay R299 per month. I did not take a device on renewal so I am not paying for a device monthly. Now Vodacom offers 20g + 20g for the same price but they do not adjust my data plan to 20g...so I am loosing out on 10g per month anytime and night time...but I still have to pay R299???