Need way to update pricing for 2012 without affecting customers on current contracts

We sell cloud computing services and bill customers using memorized SOs.   We want to update our rates (our billing is a lot like cell phone billing) for 2012 (to lower them.)  However customers are under contract for multiples of a year and their pricing is locked at the contracted levels, which may be lower or higher than the 2012 base pricing.   I'd like to update our pricing so that sales people and online purchases use the same items as 2011 pricing, but get the new prices.   Creating a second set of items (we have many items) with new prices called