Altering widget produces error, "Class name must be a valid object or a string"
I discovered that the target field was of a type that required different settings. I solved it by entering this:

$widget['active'] = '1';
$widget['#access'] = '1';
$widget['module'] = 'taxonomy';
$widget['type'] = 'taxonomy_autocomplete';
$widget['settings']['autocomplete_path'] = 'taxonomy/autocomplete';
$widget['settings']['size'] = '60';


I also realized you can go to the content type, select a field type and the underlying html for that dropdown will reveal the field type module value, and the settings under the widget dropdown will reveal the 'type' value you need to use. Hope that helps someone.