The real issue that should be in folks minds when considering purchasing one of these devices is this:
Regardless of the legality of changing the filter as it wears out, if you buy one of these devices, do not register it, and fire a single round through the filter, you have committed a Federal felony. Sure it may be easy and tempting, so is converting semi-autos to full, building out a Sten kit to work, or playing with Ammonite. You just need to ask yourself, "is it worth it?"
If you are the one unlucky bastage that someone rats out, and the law enforcement finds one of your discarded oil cans with a hole and gun powder residue, you will ruin your life.
The prisons are full of people who tried to game the system, don't be one of them.