@Override
				public void paint( Graphics g ) {
					if( inShowSnapshot || snapshot.contentsLost() )
						return;

					if( useAlpha )
						((Graphics2D)g).setComposite( AlphaComposite.getInstance( AlphaComposite.SRC_OVER, alpha ) );
					g.drawImage( snapshot, 0, 0, null );
				}
Create a snapshot of the old UI and shows it on top of the UI.
Invoke before setting new look and feel.
flatlaf-extras/src/main/java/com/formdev/flatlaf/extras/FlatAnimatedLafChange.java