how to make an existing listbox autocomplete
The list box field does not have autocomplete widget unless you use the taxonomy terms, and there are already 650 contents and you do not want to edit the content then you should use the Chosen module, the chosen module will allow you to search within the list. here is the examples page http://harvesthq.github.io/chosen/

following is the module page.
https://www.drupal.org/project/chosen