Can I register a new vehicle under the ETS before I deregister my existing vehicle?

No, the existing vehicle has to be deregistered and disposed of first. At the point when the replacement vehicle is registered, the unused COE and/or a proportion of the remainder of the vehicle’s 20-year lifespan from the existing vehicle at deregistration can then be used to compute the discounted PQP payable for the replacement vehicle.

Source: LTA