To keep the miles a user has done when they change vehicle, please see the following:
Enter a new vehicle against the user, with the new Mileage class you assigned to the vehicle.
So now this mileage class would have 0 miles assigned to it, as the user has not post any mileage expenses to that mileage class.
To then get it to match what they had done in the previous vehicle, you would find the amount done on the old vehicle, and post an a Opening Balance expense type to that vehicle. This will ensure that you can enter a expense no no values other than miles. This will then ensure that the user has miles assigned to the class, without impacting any other figures.
If you do not have a Opening Balance Expense type set up, then please do the following:
Create new Expense type, name it something distinctive that you will know refers to it purely being used as a Opening Balance type.
Change expense type to Mileage, then a tick box will appear called Opening Balance.
This can then be used in the method we have used above.