PHP reservations system new function -- 3

Hi, I have a reservation system custom built with PHP codeigniter Users create and store reservations for CARS RENTALS and these are being displayed on the calendar I want to add a new function but I am not sure how to achieve this I want when user changes the Car Number plate on the reservation and chooses to put this reservation to a different car that the system makes all the necessary changes to accommodate this reservation. The problem is that most of the times there are other reservations after the one that user is changing and these need to be removed to different cars in order to make the space required. So I need someone to come up with the idea how to solve this problem i am attaching some screen shots to have a example and of course access will be given as well Thank you