I'm on a 2 year contract for £25 a month, 700mins/free to vodafone/plus god knows how many texts...after 6 months can I go onto a cheaper tariff? I took it out when I was living with parents and didn't really use the home phone, I'm now getting my own home phone and wouldn't use the mobile as much, how do I go onto a cheaper tariff and do I pay any charges for doing it? what would they normally offer?

I also have a phone with this contract, it's not sim only!