Java Container remove method not working correctly
The Container returned by getContentPane() is, by default, the contentPane of a JRootPane managed by the top-level container, JFrame. Although, "as a convenience, the add method and its variants, remove and setLayout have been overridden to forward to the contentPane as necessary," there's no a priori way to know about the frame's internal use of component indices.
Instead, add on your own JComponent to the frame and operate on it; JPanel is a common choice.
Addendum: Also consider an alternative layout such as CardLayout, illustrated here.