Post: Upgrade Procedure for WordPress Plugin Developers

myplugin.php

<?php

define( 'MYPLUGIN_VERSION', '1.6.1' );

if ( is_admin() ) {
    include_once( plugin_dir_path( __FILE__ ) . 'includes/admin/upgrades.php' );
}

upgrades.php

<?php

function myplugin_check_upgrades() {
    $version = get_option( 'myplugin-version' );

    /**
     * Version 1.4
     */
    if ( version_compare( $version, '1.4', '<' ) ) {
        // Actions to perform
    }

    /**
     * Version 1.5
     */
    if ( version_compare( $version, '1.5', '<' ) ) {
        // Actions to perform
    }

    // Update version numbers
    if ( $version !== MYPLUGIN_VERSION ) {
        
        // Previous version installed, save prior version to db
        if ( false !== $version ) {
            update_option( 'myplugin-prior-version', $version );
        }

        update_option( 'myplugin-version', MYPLUGIN_VERSION );
    }
}
add_action( 'plugins_loaded', 'myplugin_check_upgrades' );

upgrades1.php

<?php

function myplugin_check_upgrades() {

}
add_action( 'plugins_loaded', 'myplugin_check_upgrades' );

upgrades2.php

<?php

$version = get_option( 'myplugin-version' );

upgrades3.php

<?php

/**
 * Version 1.4
*/
if ( version_compare( $version, '1.4', '<' ) ) {
    // Actions to perform
}

/**
 * Version 1.5
*/
if ( version_compare( $version, '1.5', '<' ) ) {
    // Actions to perform
}

upgrades4.php

<?php

// Update version numbers
if ( $version !== MYPLUGIN_VERSION ) {
    
    // Previous version installed, save prior version to db
    if ( false !== $version ) {
        update_option( 'myplugin-prior-version', $version );
    }

    update_option( 'myplugin-version', MYPLUGIN_VERSION );
}