Do I have to change my license plate assembly since my bulb is out on my 2004 Cadillac cts 3.6l v6

No just change the bulb .

Open the rear compartment.
Remove the rear compartment lid applique. Refer to Rear Compartment Lid Applique Replacement in Body Rear End

1.Remove the rear compartment lid trim. Refer to Rear Compartment Lid Inner Panel Trim Replacement .
Remove the fastener securing the rear compartment pull down strap to the rear compartment lid.
Remove the rear compartment pull down stap.
Remove the fastener securing the rear compartment release handle escutcheon.
Remove the rear compartment release handle escutcheon.
Remove the push-in retainers securing the rear compartment lid trim to the rear compartment lid.
Remove the rear compartment lid trim from the compartment lid.

Disconnect the rear compartment lid applique electrical connector (4).
Remove the fasteners that retain the rear compartment lid applique (1) to the rear compartment lid (2).
Remove the rear compartment lid applique (1) from the rear compartment lid (2).
Remove the rear compartment lid applique electrical harness pass through (3) from the rear compartment lid (2).

Remove the license lamp socket from the rear compartment lid applique by turning it counter clockwise.
Remove the license lamp bulb from the socket if replacing.
Remove the license lamp from the rear compartment lid applique by lifting the retaining tabs and sliding the lamp away from the applique.