Spinal Instrumentation

When a surgeon extends and existing fusion to include more levels how should the instrumentation be billed?

For example patient has a posterior fusion of C3 - C7, and the surgeon goes in and extends the fusion to levels T1 and T2. Do I bill 22842 for the instrumentation of the two additional levels? Or do I bill 22843 to include the re-instrumentation of the cervical levels? Surgeon does not replace the existing pedicle screws in the cervical vertebrae, but he does have to remove the existing rod, and shape a new one to include all the levels from C3 - T2.

I hope that makes sense to someone. Thanks for any help you can offer.