I'm sure we've all seen movies or tv shows where a guy commits a crime of some sort in his easily-recognizable BMW or GM product, and a nearby cop who cannot possibly stop the crime, but who has great eyesight, sees the getaway vehicle and radios in to dispatch the appropriate license plate number.
criminal progagonist, wise to the ways of the constabulary, presses a button or some other doodad that causes the plate to rotate, lift, drop, or slide into the body of the car, to be replaced by another license plate of similar style.
Sometimes he drives past cops with total disregard; perhaps more suspensfully, the criminal is occasionally stopped by an officer who recognizes the body style/color/make/model/driver as one wanted for a particularly heinous infraction, but because the plates don't match, lets our title-character ruffian go.
The problem, of course, is that our character can only hold so many plates in the body of his car before he has to dispose of them (running the risk of being caught with evidence in your hand/trunk), or before he simply runs out of new license plates. These are both major problems.
To solve this problem, I propose the inifinately-alterable license plate.
A high-definition printer sits to the side of and a little behind the license plate holder, two printers for states/nations with two plates.
When fleeing the crime, the printer prints (on a roll of high-tensile plastic) a new license plate number and a new state's plate template on this plastic. A roller on the opposite side of the printer pulls the old "plate" out of the holder and through the deck lid, and the new one slides sideways into place, like a printing press or film projector.
Behind the newly-printed plastic are thousands of small actuators and short rods that push out into the back of the new plastic behind the letters, to give them a bas-relief flavor (since most states have raised lettering on their plates). Presuming this is done right, these will then line up with the letters on the plate.
It's all covered with your standard perspex license-plate cover, to add to the illusion (and difficulty in detection).
Old plastic sheets with plate numbers would be cut off and fed into a small incinerator that's then filtered into the exhaust system.