Personally, I would keep the original asset number.
The history of your tests will reflect to each specific item, IF ever those results need to be viewed over, say, the last 3 years for signs of degradation in any one appliance, it helps keeping that paper trail to the original test.
You don't want to be looking for asset 10 (1st year) asset 153 (2nd year) and asset 755 (3rd year) for example.
I have the PT350 and use the automatic increments but these are specific to each customer.
e.g the customer name is say Gala Bingo. My asset increments start GB and then the appliance number GB10, GB11, GB12 etc I find this keeps it tidy and easy traceability.
Also with your Europa plus are you able to download your previous tests for that site as this will help in those retests?
Hope my ramblings are ok!!