A properly made night guard will not shift the arch that it is covering. As long as the bite is correctly set up and managed by your dentist, then there should be no shifting of the other arch either. If a night guard is not worn for a while, it is possible that the teeth that it was covering could actually shift slightly, depending upon a number of different reasons. You would know if this were the case, because the night guard would feel and fit much tighter than it had previously. Most all night guards are made of materials that are softer than teeth, and they act as a
sacrificial lamb to an extent. Meaning that the night guard itself will show the wear from clenching and/or grinding, instead of the teeth taking the brunt of that wear. So, the teeth are protected from wearing down themselves as a result of wearing the night guard. Hopefully this will serve as an answer to your question, and also give you some additional information regarding night guards and their purpose.