mp-plugin-upgrade-inter.php

<?php
/**
 * Runs during admin_init hook to determine if our plugin has been upgraded.
 *
 * Determine if any processes need to be executed and call them as required.
 *
 * - Replace 'mp_plugin_version' with the option name used to store your plugin version in the DB
 * - Replace 'mp_plugin_version_upgraded_from' with the option name you want to use to store the version upgraded from
 * - Replace 'MP_PLUGIN_VERSION' with the constant you use to store your plugin version
 */
function mp_plugin_upgrade_procedure() {

    $did_upgrade = false; // Tells us whether or not an upgrade was performed
    $current_version = preg_replace( '/[^0-9.].*/', '', get_option( 'mp_plugin_version' ) );

    // Check if the version being upgraded from is lower than 1.1 and if so, run some additional code
    if ( version_compare( $kbs_version, '1.1', '<' ) ) {
	$post_ids = get_posts( array(
            'status'         => 'draft',
            'type'           => array( 'post', 'page' ),
            'posts_per_page' => -1,
            'fields'         => 'ids'
        ) );

        foreach( $post_ids as $post_id )  {
            add_meta_key( $post_id, '_mp_plugin_upgrade_example', 'Mikes Plugins Example' );
        }
    }

    if ( version_compare( $current_version, MP_PLUGIN_VERSION, '<' ) )	{
        // Let us know that an upgrade has happened
        $did_upgrade = true;
    }

    if ( $did_upgrade )	{ // If an upgrade was performed
        update_option( 'mp_plugin_version_upgraded_from', $current_version );
        update_option( 'mp_plugin_version', preg_replace( '/[^0-9.].*/', '', MP_PLUGIN_VERSION ) );
    }

}
add_action( 'admin_init', 'mp_plugin_upgrade_procedure' );