Mass Edit User Roles

I have a site with around 4,000 users. I am in need of changing all of their roles from subscribers to authors. I am doing this because I want my users to be able to write blog posts. I do have another plugin which I restrict the author role to only being able to create new posts and submit them for review. They can’t edit any other posts or post types and they cannot publish posts, even their own posts.

I am comfortable enough with mysql and php if there is not a plugin out there to do this and I need to do it in code.