Trigger workflow on exisitng list items created prior in SharePoint 2010

I created a workflow on an existing list in SharePoint 2010 to send notification to users to remind them the item expiration date.

The workflow works fine, but the problem is the list already has 600 items, and the workflow can only be triggered when item is created or modified or manually.    

How can I get the workflow to run without having to open each item one by one to save it (an modification action) in order to trigger it?

Is there a way to get workflow triggered on existing items?

Thanks for any tips!