First things first. A couple of the people above seem to think that Council Tax banding is something to do with the your council. It isn't - the Valuation Office Agency decide.
Secondly, please indicate whether your house is being singled out in this way - it seems most unlikely, and I assume ALL your neighbours are similarly impacted?
Thirdly I do not believe you will have to pay anything back.
Fourthly, your successful attempt at asking the VOA for a reduction may have been the trigger that helped them spot a mistake in all the surrounding properties, and yes, I do believe that properties can be re-assessed if a mistake has occurred. If you look here you will see that an appeal may result in one of three outcomes - no change, a change up or a change down.
http://www.voa.gov.uk/news/information/your_co uncil_tax_band.htm
In England you can appeal the new banding within 3 months. See here
http://www.voa.gov.uk/council_tax/appeals_dire ct.htm