not sure why you would want to do this but you must have your reasons. It can be done in 2 ways in a roundabout way if you have a while to wait.
the first way obviously the most simplest is to change your id but your old id will be for all to see for about a month b4 the new id takes over completly but you can still be traced then through your old id.
Another way is to start up a completely new account then get a few ratings and transfer your old account to the new one and then you will have all your ratings of your old account mixed into your new one.
As I said not sure why you would want to do this but can think of many reasons that are dodgy but many more that are not.....