Replacing Instrument cluster

The message centre on my instrument cluster has an increasingly bright background. I decide to buy a used cluster and swap over the message display. In HK the speedo is only in KPH, so I can't swap to a cluster with MPH/KPH as it won't pass the MOT. Go figure!!!!

Does anyone know if my original mileage info is stored on the two chips on the message display PCB, or on the ECM/BeCM?

Thanks.

The car will display whichever mileage is the higher from the cluster or BECM.