Simply dump the snippet of code below into a file in your WordPress sites mu-plugins folder. This will add a simple statement to the top of the admin panel, alerting the contributors to your site that any edits they make will be lost due to the site undergoing a move to another server.

<?php

/**
 * Show admin notification
 *
 * @author Ryan Hellyer <ryanhellyer@gmail.com>
 */
function show_moving_message() {
	// Shows as an error message. You could add a link to the right page if you wanted.
	echo( '
		 <div id="message" class="error">
			<p>
				<strong>Notice:</strong> We are in the process of moving <strong>' . str_replace( 'http://', '', home_url() ) . '</strong> to another server. It is possible that any posts you create from now until the move is complete may be lost. Please wait until this notice disappears before editing your website.
			</p>
			<p>
				The move <em>should</em> be completed within the next few hours. 
				Sorry for the inconvenience!
			</p>
		</div>' );
}
add_action( 'admin_notices', 'show_moving_message' );

?>

server-move-notification