Custom Field Image to Featured Image

Hi,

I’ve been all over the forums (and Google) trying to find an answer to this problem. I’ve seen plenty of people asking the same thing, with no definitive answer.

I’m moving a blog from one theme to another. The original blog has the post’s “featured image” in a custom field called “posts settings”. The new site has the featured image in WordPress’ default “Featured image”.

When I export the .xml file from the old blog and try to import it to the new one, I don’t get any “Featured images”. I know that this is because they have different “meta_keys”.

The old blog has:

<wp:meta_key>mainimg_medium</wp:meta_key>
<wp:meta_value><![CDATA[https://imageurl.com]]></wp:meta_value>

and the new one has:

<wp:meta_key>_thumbnail_id</wp:meta_key>
<wp:meta_value><![CDATA[image ID number]]>

I can’t just find and replace “mainimg_medium” with “_thumbnail_id” because “_thumbnail_id” doesn’t recognize URLs, it only recognises ID numbers.

Any help will be greatly appreciated,
Thanks!