You can retain a user's cumulative mileage when they change vehicle by posting an Opening Balance expense against the new vehicle.
Enter a new vehicle against the user with the correct mileage class.
📌 Note: The new mileage class starts at zero miles until an Opening Balance is posted against it.
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 an Opening Balance expense type, create one as follows.
Create a new expense type with a distinctive name to indicate it is used only as an Opening Balance.
Change the expense type to Mileage. A checkbox labelled Opening Balance will appear — select it.
Use this expense type to post the Opening Balance as described above.