import javax.swing.JFrame;

public class Main {
	public static void main(String[] args) {
		Start window = new Start();
		window.setVisible(true);
	}
}

import java.awt.BorderLayout;
import java.awt.Container;
import java.awt.Dimension;
import java.awt.GridBagConstraints;
import java.awt.GridBagLayout;
import java.awt.Insets;
import javax.swing.*;

public class Start extends JFrame {

	Start (){
		super ("Name_window");
		setDefaultCloseOperation(EXIT_ON_CLOSE);
		setLocationRelativeTo(null);
			
		JPanel bigPanel = new JPanel(new BorderLayout());
		JPanel bigPanel2 = new JPanel(new GridBagLayout());

		Container cont = this.getContentPane();
		cont.add(bigPanel);
		
		JButton ok = new JButton("OK");
		ok.setPreferredSize(new Dimension (110, 25));
		
		ActionOkStart action = new ActionOkStart ();
		ok.addActionListener(action);
		bigPanel.add(ok, BorderLayout.CENTER);
		
		bigPanel2.add(new JLabel("Добро подаловать!"), new GridBagConstraints(0, 0, 1, 1, 0, 0, GridBagConstraints.CENTER, GridBagConstraints.NONE, new Insets (15,5,5,5), 0, 0));
		pack();
	}
}

import java.awt.event.ActionEvent;
import javax.swing.AbstractAction;

public class ActionOkStart extends AbstractAction {

	@Override
	public void actionPerformed (ActionEvent arg0) {
//	как при нажатии убрать bigpanel из cont и добавить  bigpanel2
	}
}