It will cost u $1300 over at LTA side. Go to LTA and perform a number retention which will cost you $1300. After which ur existing car will be assigned with a new registration number. You will need to make a new car plate for your car.
When u buying the pre own car, tell the dealer that you wish to use your retained number. I think they might charge u an admin fee if not you can always do it yourself at LTA