Final answer:
A rigid abdomen and vomiting in a patient with a gastric ulcer indicate a possible medical emergency and require an urgent change in the nursing care plan.
Step-by-step explanation:
The assessment of a 72-year-old patient with a gastric ulcer that warrants an urgent change in the nursing plan of care is a rigid abdomen and vomiting following indigestion. This symptom could be indicative of a serious complication such as perforation or peritonitis, which are medical emergencies requiring immediate attention. While the other symptoms listed may be associated with the patient's known gastric ulcer, they do not suggest an acute, life-threatening condition like a rigid abdomen does.