It will show whatever milage the old car was at (40k).
You could try contacting VDO OEM services and see if they would
reset your mileage for a fee.
Also, each dealership servce department should have a signal
generator designed to connect to the cluster and run the miles forward. Sadly, when turned up to max, (160 mph) the cluster
would take forever to add 20k. About 125 hours.
The biggest thing is to make sure 96 and 98 year clusters are
even compatible. (same connectors, functions, ect.....)
and keep in mind, a 98 230 cluster will have an active FSS program running even thought your 96 didn't use Flex Service.