Can your upper jaw shift back to being narrow after a palate expander treatment as a teenager if you don't wear your retainers for braces?

Depends: Some palatal expanders produce more stability than others. Retainers prevent teeth from shifting, but if underlying bone is not the appropriate size/shape, tooth movement can appear exaggerated. I assume you're asking the question because you have noticed some movement. Best to return to your Orthodontic Specialist, the professional that knows you best, for reevaluation before serious relapse.