Why must old currency be taken out of circulation when new currency is made?

Old currency must be taken out of circulation to prevent confusion and ensure the integrity of the monetary system. Retiring outdated notes helps combat counterfeiting and reduces the risk of fraud, as newer designs often incorporate advanced security features. Additionally, it streamlines transactions and accounting processes, making it easier for businesses and consumers to use the most current and secure forms of money. Finally, removing old currency can help central banks better manage the money supply and inflation.