Wordpress Activation/Upgrade Hook Helper

WpPluginUpgradeBase.php

<?php

if(!class_exists('WpPluginUpgradeBase')) :

abstract class WpPluginUpgradeBase {

	function __construct() {
		add_action( 'admin_init', array(&$this, 'load_plugin') );
	}

	/**
	 * Namespace the given key
	 * @param string $key the key to namespace
	 * @return the namespaced key
	 */
	public function N($key = false) {
		// nothing provided, return namespace
		if( ! $key || empty($key) ) { return get_class($this); }
		return sprintf('%s_%s', get_class($this), $key);
	}
	
	const HOOK_ACTIVATED = 'activated';
	const HOOK_LOADED = 'loaded';
	
	public function loaded_hook_name() { return $this->N(self::HOOK_LOADED); }
	public function activated_hook_name() { return $this->N(self::HOOK_ACTIVATED); }

	public function register($original_file) {
		register_activation_hook( $original_file, array( &$this, 'activate' ) );
	}

	public function activate() {
		// delay doing anything until plugins are actually ready
		// see http://codex.wordpress.org/Function_Reference/register_activation_hook#Process_Flow
		add_option( $this->N(), $this->N() );

		/* other non-plugin-dependent activation code here */
		do_action( $this->activated_hook_name(), 'activated' );
	}

	public function load_plugin() {

		if ( is_admin() && get_option( $this->N() ) == $this->N() ) {

			// clear the 'run once' flag
			delete_option( $this->N() );

			/* do stuff once right after activation */
			do_action( $this->loaded_hook_name(), 'loaded' );
		}
	}
}//---	class	WpPluginUpgradeBase


endif; // class_exists

YourPlugin.php

<?php
// the main plugin file
// header....etc...then...

// handle plugin upgrades
// http://codex.wordpress.org/Function_Reference/register_activation_hook#Examples
include_once dirname( __FILE__ ) . '/WpPluginUpgradeBase.php'; // core helper
include_once dirname( __FILE__ ) . '/YourPluginUpgrader.php'; // your custom code
$ugrader = new YourPluginUpgrader();
$ugrader->register(__FILE__);

class YourPlugin {
  const pluginVersion = '1.6.0'; // to make it easier to compare in upgrade check, rather than calling `get_plugin_data` http://codex.wordpress.org/Function_Reference/get_plugin_data
  
  // other plugin code
}

YourPluginUpgrader.php

<?php
class YourPluginUpgrader extends WpPluginUpgradeBase {

	function __construct() {
		parent::__construct();
		
		add_action($this->loaded_hook_name(), array(&$this, 'loaded'));
		
		//add_action($this->activated_hook_name(), array(&$this, 'test'));
	}
	
	public function test($action) {
		// just prove it was called
		error_log(print_r(array(__FILE__, __CLASS__, __FUNCTION__, $action), true));
	}

	
	public function loaded($action) {
		// check current plugin version
		
		// compare against prev version and do stuff
	}
}

YourPluginUploaderWithSteps.php

<?php

// an example that goes through upgrade steps depending on how out-of-date

class YourPluginUpgradeSteps extends WpPluginUpgradeBase {

	function __construct() {
		parent::__construct();
		
		add_action($this->loaded_hook_name(), array(&$this, 'loaded'));

		## just to show that each hook was hit
		## add_action($this->loaded_hook_name(), array(&$this, 'test'));
		## add_action($this->activated_hook_name(), array(&$this, 'test'));
	}
	
	public function test($action) {
		// just prove it was called
		error_log(print_r(array(__FILE__, __CLASS__, __FUNCTION__, $action), true));
	}

	/**
	 * List of important upgrade steps
	 */
	var $upgrades = array(
		self::VERSION_FEATURE_A_ADDED,
		self::VERSION_WHEN_THIS_HAPPENED,
		self::VERSION_WHEN_THAT_HAPPENED
		);
	const VERSION_FEATURE_A_ADDED = '1.2.0';
	const VERSION_WHEN_THIS_HAPPENED = '1.2.2';
	const VERSION_WHEN_THAT_HAPPENED = '1.5';
	
	public function loaded($action) {
		// check current plugin version
		$current = YourPlugin::pluginVersion;
		
		// compare against prev version and do stuff
		$prev = get_option( 'YourPlugin_version' );

		// special case: we've never set the version before; not all plugins will need to upgrade in that case
		if(empty($prev) || version_compare($prev, $current) < 0) {
			// are there upgrade steps depending on how out-of-date?
			foreach($this->upgrades as $next_version) {
				if(version_compare($prev, $next_version) < 0) $this->do_upgrade($prev, $next_version);

				$prev = $next_version;
			}
		}

		// update stored plugin version for next time
		update_option('YourPlugin_version', $current);
	}

	function do_upgrade($prev, $next) {
		## error_log('upgrade from ' . $prev . ' to ' . $next);
		switch($next) {
			case self::VERSION_FEATURE_A_ADDED:
				## error_log('doing feature A upgrade...');
				break;
			case self::VERSION_WHEN_THIS_HAPPENED:
				## error_log('doing feature B upgrade...');
				break;
			case self::VERSION_WHEN_THAT_HAPPENED:
				## error_log('doing feature C upgrade...');
				break;
		}
	}
}

You could probably replace register_activation_hook with hooking to plugins_loaded only when is_admin per discussions at http://wordpress.stackexchange.com/questions/144870/wordpress-update-plugin-hook-action-since-3-9/160749#160749