The plan may provide that postpetition contractual payments on vehicles shall be made directly to the creditor. All such direct payments shall be made as they come due postpetition. The plan shall provide that arrearages will be paid through the chapter 13 trustee. If the plan changes the amount of payment or duration due to the fact that any portion of the claim is deemed unsecured, then all payments so provided must be paid through the chapter 13 trustee.