Custom field > Featured image

For years, I’ve used custom fields for a featured image and caption for my posts. I now want to migrate to using WordPress’ built-in Featured Image function.

Is there a plugin or a simple SQL query that I can use to easily convert my custom fields (postimg_url & postimg_caption) into Featured Images and image captions respectively?

And once I’ve done so, what code do I swap in to replace my current custom field calls, in order to pull the images and captions back into my templates?

Thanks