Many students with Asperger profiles get distressed when anything changes in their day. Alerting them in writing that a change is anticipated can help them adjust. You can help reassure them that everything will still be OK and help them understand what they will be doing instead of their regular schedule. Try using this model or one of your own. See if students can name the unexpected change. Some like to call these changes other names like, “oops”, “zigzag”, or “switch time.”
You are helping your students learn that they can manage changes in routines and that these last minute surprises happen to everyone.