Changing the layout of a vehicle can be done if there are no inspections made for that vehicle.
However, if there are, then you will have noticed that the menu "Vehicle Layout" is grayed out and cannot be changed.
This restriction has been put in place to avoid corruption of data. For instance, if there are test tyres for this vehicle currently on the vehicle, changing its layout may make some mounting position invisible. You would therefore be unable to demount these tyres. They would remain forever associated with the vehicle and affect the quality of your data.
It may also affect data collected by other team members (ex: Field Engineering data in TTMS).
There is however a solution: if you want to change the layout, simply delete all the inspection data that may be associated with it (In Service and Tracker). You will then be able to change the layout.