Convert users to guest authors

So I have several hundred regular users on my site with profiles and the like. Some with access to my dashboard. Is there an effective and easy way to convert these users to Guest Authors? I recently tweaked Guest Author profiles so I want to rely on these for highlighting authors.

I’d be fine with creating new guest author profiles then deleting there old profiles but then when I do that their work is deleted. It’s a huge hassle.

Fantastic plugin otherwise.