The icons don't change unless you initiate the change. IconPackager is not a program that is always running the background, "skinning" your icons. It simply applies a package when instructed, freeing you from the task of changing them one, by one. Once the trail expires, you no longer have the option of applying packages, but the icons will not revert to default.....
If you decide to buy the programs, listen to aimzzz .... an Object Desktop subscription is the way to go.