Change post author field type

It’s possible, but not simple to accomplish. I’m unaware of a plugin to accomplish it, but it’d be worth doing some searching. Developing your own solution would involve using jQuery to replace the current field with a searchable field. There could be a slight delay after the page loads before the desired field appears.

Replacing one field with another is fairly straight forward. It’s developing the search functionality where most of your efforts would be. Some sort of auto-fill app could likely be adapted to handle this.