Does "sealed battery" mean it cannot be replaced, so when it dies a new unit must be purchased?

The battery lasts 10 years unless it is defective. The detectors in smoke alarms are only good for 10 years (the radioactive sample in them decays and becomes too weak), so replacing the unit would be required after 10 years anyway. Check the labels on your smoke alarms and replace them if they are 10 years old or older.