Migrating a website using custom image tags to use featured images instead

Question

I have a site that was built using a Wordpress 2.x era theme that uses a custom field "image" to hold the URL to use as a featured image for each post.

I am migrating to use the PageLines Platform Pro theme, which uses the now standard Featured Images feature - and I am looking for a way to automatically migrate the thousands of old posts to work with this new theme without losing their featured image.

This other similar Stack Exchange question has an accepted answer that solves this problems by using the Get The Image plugin to fall back to a custom field if there is not a featured image set.

The problem with Get The Image however is that it requires modifying the theme to add a function call. This will break with every upgrade to PageLines, and makes it harder to change themes in the future.

I am looking for a way to do a one-time migration, but I haven't found any simple instructions how to manage this. Any ideas??

UPDATE: This tutorial seems to indicate one way to do this: http://pmg.co/migrating-post-thumbnails-on-old-wordpress-themes

Is this the best way? Are there any other techniques I should pursue?

Answer 1

An extremely simple way is to make a Child Theme of the theme that you want to use, and still use Get the Image in the functions.php file of that child theme.

You should probably be doing this anyways if you are editing any files in the theme. It makes it super easy to upgrade commercial and public themes.