The legacy instruments are less CPU- and memory-intensive versions of equivalent Logic Pro X instruments. All legacy instruments feature a few carefully chosen parameters that provide maximum impact and flexibility, making it easy to create great sounds.
Legacy instruments are automatically loaded when a GarageBand project is imported, or a Logic project or MainStage concert that uses these instruments is opened.
Press Option, then click the instrument slot on a channel strip.
The plug-in menu opens, with a Legacy submenu shown below Vintage Electric Piano.
Choose the legacy instrument plug-in that you want to insert from the Legacy submenu.