To simplify reporting requirements in relation to company cars, it is no longer necessary to notify HMRC when an employee with a company car changes his or her car. Where the cash equivalent of the replacement is different to the original the employee's tax code will not be adjusted until the P11D has been submitted. This may result in an over or underpayment of tax.
To remedy this, from April 2011 it will be possible to notify HMRC electronically where an employee with a company car is issued with a replacement. However as the current P46(Car) does not accommodate this and there are no plans to change the form, such notification can only be made electronically.
Further details are available on the HMRC website.