How to change a user reference field from select list to autocomplete in a Views exposed filter?
To get User Reference Field as autocomplete exposed filter,


Let me consider you have created a user reference field in your content type as Users-ref 


Now in your View :


Add a relationship for your entity reference field for Eg: Entity Reference: Users-ref and make it required.
Now add a filter criteria User:Name  with the relationship Entity Reference: Users-ref
you should find (User entity referenced from field_users_ref) User: Name (exposed) in your filter criteria now.
And this should be an autocomplete widget!!


There is also a module named Views Autocomplete Filters along with this wonderful tutorial!!