Automatically Thumbnail Old and Current Posts via Custom Field?

Hello,

I have been using a theme that didn’t support thumbnails on the home pages for over three years. I now have a massive amount of content without thumbnail posts.

I want to switch to a theme that does support thumbnails. Is there a way to automatically generate thumbnails for all of my wordpress posts based on the first image in any given post to accomplish this? I really like some of the modern looks of these types of themes but it would be impossible to go back and custom field thumbnails one-by-one.

If anyone knows of a solution for this, I would really appreciate it?

Thank you!