zaarin_2003:
Several issues here.
You can always ask but the issue is not that you are in credit but whether the monthly payment is right for your annual consumption. It's perfectly reasonable (and correct) for there to be a surplus ahead of the winter heating season. If you ask, that's the calculation NPower will do. I suggest you read the NPower website where it describes monthly direct debit.
Next you should use a Consumer Focus accredited website to compare your current tariff with others available using your actual annual consumption, but be aware it is quite likely that on headline cost NPower will be amongst the cheapest. However, if you want and you have indicated that it is important to you, you can also compare on customer rating.
If you switch, in due course to would receive any closing balance back from Npower. Be aware however that most NPower tariffs have substantial discounts (>£100) paid 12 months in arrears and you would lose that if you did not delay until after any discount was due.