When the COE on your car expires, you will have to make a decision on what you are going to do with your car. The three choices you have are:
If you decide to export your car (because it's not worth a new COE but too valuable to be scrapped), please visit the One.Motoring website for a detailed overview of the process and required documentation.
Do make sure that the car will be allowed on the road in the country to which you are exporting. Regulations vary from country to country and it would be a shame if you spend so much money on exporting the car, only to find out you will need to invest a lot of money on modifying the car to get in on the road in the country to which you exported.