Delaware Law (Title 21 Section 2507) requires that the owner(s) of a Delaware registered vehicle(s) that moves to another state and has the vehicle(s) registered into that state must return the Delaware plate(s) with a statement that the vehicle(s) are now registered in the new state to include the name of the state to:

Division of Motor Vehicles
Attn: Uninsured Motorist Section
P.O. Box 698
Dover, DE 19903