You'll need to head to your dealer, the vehicle will need reprogramming in order to recognize the new cluster, the mileage is stored in the IPC(Instrument Panel Cluster).
Until you do that, the new cluster will remain frozen at the mileage of the vehicle it was removed from, when reprogrammed the vehicle and cluster will able to communicate and it will reflect your actual mileage.
Call around to a trusted local garage specializing in GM cars, a technician armed with a "Tech 2" scan tool is capable of doing the reprogramming. You really should have an accurate odometer reading for registration renewals and state safety inspections, etc.
The vehicle odometer is calculated and stored electronically in the body control module (BCM) and is sent via the serial data circuit to the instrument panel cluster (IPC) for display. The BCM calculates and stores the mileage based on the vehicle speed serial data messages from the engine control module (ECM). The IPC displays the season odometer information from the BCM in the driver information center (DIC). If the stored VIN in the IPC matches the stored VIN, the odometer data from the BCM will then be displayed. If the VINs do not match, the IPC will freeze the odometer value for the entire ignition cycle.
We're going to sell my wife's HHR. It would be nice to have the mileage correct. I took it in the dealer they had it a week and told me it was fixed. Not fixed because they had down the speedometer wasn't working. I blame the old guy in the service dept.
So I got a call today and what their tech has "discovered" is that until the mileage in the BCM matches the mileage in the cluster that the odometer won't work. Also they can't read the mileage in the BCM. WTF? With my interactions with the guy on the phone I tend to trust the answers I get her a lot more. Should I call another Chevy dealership? Try to find a local shop(we just moved here)? Or send the old cluster off to be repaired maybe?
I called another Chevy dealer here in town and talked to service there. The girl I talked to seemed on the ball and knew exactly what I was talking about and said it wouldn't take more than an hour or so. Now it's time to make sure I get my money back from the other dealership since I paid the first time they told me it was fixed!