Changing article text and summary with template_preprocess_node()
I believe that the node has already been rendered by the time template_preprocess_node() is called so changing any values on the node object will have no effect. You can try looking at $variables['node']-&gt;content to see if you can modify the text in that (I think this is the full HTML rendered node though, so lots of extra text/markup to deal with here).

You're probably better off using hook_node_load() instead. This gives you a chance to alter the node object directly before it gets rendered.