[vantage][4.0] Serial Numbers

Does anyone use serialnumbers and the service module?

We are facing the following problem:
We sell a truck to customer AAA with a 3 year service contract.
The Truck has a serialnumber: 50-100-0704

After three years AAA sells its truck to BBB.
BBB wants a service contract with us for this particular Truck with
the serialnumber: 50-100-704.

I am unable to enter a service contract for BBB for truck 50-100-704.
Does anyone has a workaround?