Alt/Option-drag a Project item onto a Timeline layer, or press Ctrl+Alt+/ (Cmd+Opt+/) with layers and an item selected, to swap the layer's source. The layer keeps its name, timing, transforms, keyframes, expressions, effects, masks, switches, parenting and track matte; a source with audio adds Audio properties when missing. Precompositions can't be nested inside themselves.
AVLayer.replaceSource now runs the same engine command and closes the replaceSource part of #813.