Tanks can certainly be retested. It's called a hydro static test and can only be done by certified CO2 dealers or fire equipment supply companies. I have my tanks filled at a fire equipment company that tests them so when they are out of date, they just keep them and I get them back when done. Pay for it of course.
A couple of my tanks belong to the CO2 company. They charge a deposit but the tank can be exchanged and it is their problem to keep them tested. Many of them do not care if it is their tank and just swap it out. Depends on how anal they are about it.
I had one that was a know defective model. The CO2 guy removed the valve and beat the top so another one could not be attached. Gave me an exchange tank to use without a deposit though.