Ok - I found the solution. To trigger the
onContentChangeState
event from inside another plugin, on an article with for example id=4:
use Joomla\CMS\Factory;
use Joomla\CMS\CMSObject;
use Joomla\CMS\Plugin\PluginHelper;
$article[0] = 4;
PluginHelper::importPlugin('content');
$app = Factory::getApplication();
$test = $app->triggerEvent('onContentChangeState', array('com_content.article', &$article, &$params, 0));